Undeclared first use in this function ошибка c - Решение и исправление самых разных ошибок на TopOshibok.ru

In your code

 temp = (Node*) malloc(sizeof(struct Node));

should be

 temp = malloc(sizeof(struct Node));

or, for better,

 temp = malloc(sizeof *temp);

eliminating the issue, as you need not to cast the return value of malloc() and family in C..

The error was, however, because you missed the struct keyword there.

To address the third error,

«expected declaration or statement at end of input.»

well, you missed the ending } for the print() function.

FWIW, you should always check the validity of the returned pointer by malloc() and family of functions before making use of it.

Having said that,

For the first one I’ve searched and found out it’s because of C89 and I have to declare all the variables at the top of my functions

is not true. Yes, in C89, you needed to declare all the variables at top, but that did not allow you to skip the struct keyword, anyway.

Источник

Undeclared errors, such as the «first use in this function» issue, are common when programming in C, C++, or other languages that require explicit variable declarations. This guide will help you understand why these errors occur and how to fix them step-by-step.

Table of Contents

Understanding Undeclared Errors
Step-by-Step Guide to Fixing ‘First Use in This Function’ Issues
Related Links
FAQs

Understanding Undeclared Errors {#understanding-undeclared-errors}

An undeclared error occurs when the compiler encounters a variable or function that has not been declared before its first use. In languages like C and C++, you must declare a variable or function before using it. Failing to do so will result in a compilation error, such as the «first use in this function» error.

This error generally occurs due to:

Typographical errors in variable or function names.
Missing or incorrect variable or function declarations.
Misplacement of declarations (e.g., declaring a variable inside a loop or block).
Inclusion issues with header files.

Step-by-Step Guide to Fixing ‘First Use in This Function’ Issues {#step-by-step-guide}

Follow these steps to resolve the «first use in this function» error in your code:

Step 1: Identify the Error

First, locate the error message in your compiler output. It should look similar to the example below:

error: 'variable_name' undeclared (first use in this function)

Take note of the variable or function name causing the error.

Step 2: Verify Spelling and Capitalization

Ensure that the variable or function name is spelled correctly and that the capitalization is consistent throughout your code. A common cause of undeclared errors is a mismatch in variable or function names due to typos or inconsistent capitalization.

Step 3: Check for Missing or Incorrect Declarations

Verify that the variable or function has been declared before its first use. If the declaration is missing, add an appropriate declaration before the first use of the variable or function.

For example, if you’re using a variable named count of type int, ensure that it is declared as:

int count;

Step 4: Ensure Proper Declaration Placement

Make sure that the variable or function declaration is placed correctly in your code. Variables should generally be declared at the beginning of a function or block. Functions should be declared before their definitions or in a separate header file.

If the variable or function is defined in a header file, ensure that the header file is included in the source file where the variable or function is being used. Use the #include directive to include the header file, as shown below:

#include "header_file.h"

Additionally, verify that the header file itself is correctly formatted and that the variable or function declaration is present and accurate.

C Programming: Understanding Variable Scope and Storage Class
C++ Programming: Understanding Variable Scope and Storage Class

FAQs {#faqs}

What is an undeclared error? {#what-is-an-undeclared-error}

An undeclared error occurs when the compiler encounters a variable or function that has not been declared before its first use. This results in a compilation error, such as the «first use in this function» error.

Why do undeclared errors occur? {#why-do-undeclared-errors-occur}

Undeclared errors generally occur due to typographical errors in variable or function names, missing or incorrect variable or function declarations, misplacement of declarations, or inclusion issues with header files.

How do I fix an undeclared error? {#how-do-i-fix-an-undeclared-error}

To fix an undeclared error, follow these steps:

Identify the error.
Verify spelling and capitalization.
Check for missing or incorrect declarations.
Ensure proper declaration placement.
Check header file inclusions.

How do I declare a variable or function in C/C++? {#how-do-i-declare-a-variable-or-function}

In C/C++, you need to specify the data type and the variable name for variable declarations. For example:

int myVariable;

For function declarations, you need to specify the return type, function name, and the argument list. For example:

int myFunction(int a, int b);

What is the scope of a variable? {#what-is-the-scope-of-a-variable}

The scope of a variable refers to the region of the program in which the variable can be accessed. In C/C++, there are four different storage classes that determine the scope and lifetime of a variable: auto, static, extern, and register.

Источник

Types of Errors in C

Overview

An error in the C language is an issue that arises in a program, making the program not work in the way it was supposed to work or may stop it from compiling as well.
If an error appears in a program, the program can do one of the following three things: the code will not compile, the program will stop working during execution, or the program will generate garbage values or incorrect output. There are five different types of errors in C Programming like Syntax Error, Run Time Error, Logical Error, Semantic Error, and Linker Error.

Scope

This article explains errors and their types in C Programming Language.
This article covers the explanation and examples for each type of error in C Programming Language (syntax error, run time error, logical error, semantic error, linker error).

Introduction

Let us say you want to create a program that prints today’s date. But instead of writing printf in the code, you wrote print. Because of this, our program will generate an error as the compiler would not understand what the word print means. Hence, today’s date will not print. This is what we call an error. An error is a fault or problem in a program that leads to abnormal behavior of the program. In other words, an error is a situation in which the program does something which it was not supposed to do. This includes producing incorrect or unexpected output, stopping a program that was running, or hindering the code’s compilation. Therefore it is important to remove all errors from our code, this is known as debugging.

How to Read an Error in C?

In order to resolve an error, we must figure out how and why did the error occur. Whenever we encounter an error in our code, the compiler stops the code compilation if it is a syntax error or it either stops the program’s execution or generates a garbage value if it is a run time error.

Syntax errors are easy to figure out because the compiler highlights the line of code that caused the error. Generally, we can find the error’s root cause on the highlighted line or above the highlighted line.

For example:

#include <stdio.h>
int main() {
    int var = 10
    return 0;
}


Output:
error: expected ',' or ';' before 'return'
      4 |  return 0;


As we can see, the compiler shows an error on line 4 of the code. So, in order to figure out the problem, we will go through line 4 and a few lines above it. Once we do that, we can quickly determine that we are missing a semi-colon (;) in line 4. The compiler also suggested the same thing.
Other than the syntax errors, run time errors are often encountered while coding. These errors are the ones that occur while the code is being executed.
Let us now see an example of a run time error:



  


                       
  




#include<stdio.h>

void main() {
    
    int var;
    var = 20 / 0;
    
    printf("%d", var);
}


Output:
warning: division by zero [-Wdiv-by-zero]
    6 |     var = 20 / 0;


As we can see, the compiler generated a warning at line 6 because we are dividing a number by zero.
Sometimes, the compiler does not throw a run time error. Instead, it returns a garbage value. In situations like these, we have to figure out why did we get an incorrect output by comparing the output with the expected output. In other cases, the compiler does not display any error at all. The program execution just ends abruptly in cases like these.
Let us take another example to understand this kind of run time error:
#include <stdio.h>
#include <stdlib.h>

int main() {
    
	int arr[1]; 
	arr[0] = 10; 

	int val = arr[10000]; 
	printf("%d", val); 
    return 0;
}


Output:

In the above code, we are trying to access the 10000th element but the size of array is only 1 therefore there is no space allocated to the 10000th element, this is known as segmentation fault.

Types of Errors in C

There are five different types of errors in C.

Syntax Error
Run Time Error
Logical Error
Semantic Error
Linker Error

1. Syntax Error

Syntax errors occur when a programmer makes mistakes in typing the code’s syntax correctly or makes typos. In other words, syntax errors occur when a programmer does not follow the set of rules defined for the syntax of C language.

Syntax errors are sometimes also called compilation errors because they are always detected by the compiler. Generally, these errors can be easily identified and rectified by programmers.

The most commonly occurring syntax errors in C language are:

Missing semi-colon (;)
Missing parenthesis ({})
Assigning value to a variable without declaring it

Let us take an example to understand syntax errors:

#include <stdio.h>

void main() {
    var = 5;    // we did not declare the data type of variable
     
    printf("The variable is: %d", var);
}


Output:
error: 'var' undeclared (first use in this function)


If the user assigns any value to a variable without defining the data type of the variable, the compiler throws a syntax error.
Let’s see another example:
#include <stdio.h>

void main() {
    
    for (int i = 0;) {  // incorrect syntax of the for loop 
        printf("Scaler Academy");
    }
}


Output:
error: expected expression before ')' token


A for loop needs 3 arguments to run. Since we entered only one argument, the compiler threw a syntax error.
2. Runtime Error
Errors that occur during the execution (or running) of a program are called RunTime Errors. These errors occur after the program has been compiled successfully. When a program is running, and it is not able to perform any particular operation, it means that we have encountered a run time error. For example, while a certain program is running, if it encounters the square root of -1 in the code, the program will not be able to generate an output because calculating the square root of -1 is not possible. Hence, the program will produce an error.
Runtime errors can be a little tricky to identify because the compiler can not detect these errors. They can only be identified once the program is running. Some of the most common run time errors are: number not divisible by zero, array index out of bounds, string index out of bounds, etc.
Runtime errors can occur because of various reasons. Some of the reasons are:

Mistakes in the Code: Let us say during the execution of a while loop, the programmer forgets to enter a break statement. This will lead the program to run infinite times, hence resulting in a run time error.
Memory Leaks: If a programmer creates an array in the heap but forgets to delete the array’s data, the program might start leaking memory, resulting in a run time error.
Mathematically Incorrect Operations: Dividing a number by zero, or calculating the square root of -1 will also result in a run time error.
Undefined Variables: If a programmer forgets to define a variable in the code, the program will generate a run time error.

Example 1:
// A program that calculates the square root of integers
#include <stdio.h>
#include <math.h>

int main() {
    for (int i = 4; i >= -2; i--)     {
        printf("%f", sqrt(i));
        printf("n");
    }      
    return 0;
}


Output:
2.000000
1.732051
1.414214
1.000000
0.000000
-1.#IND00
-1.#IND00


**In some compilers, you may also see this output: **



  


                       
  




2.000000
1.732051
1.414214
1.000000
0.000000
-nan
-nan


In the above example, we used a for loop to calculate the square root of six integers. But because we also tried calculating the square root of two negative numbers, the program generated two errors (the IND written above stands for «Indeterminate»). These errors are the run time errors.

-nan is similar to IND.
Example 2:
#include<stdio.h>
 
void main() {
    int var = 2147483649;

    printf("%d", var);
}


Output:

This is an integer overflow error. The maximum value an integer can hold in C is 2147483647. Since in the above example, we assigned 2147483649 to the variable var, the variable overflows, and we get -2147483647 as the output (because of the circular property).
3. Logical Error
Sometimes, we do not get the output we expected after the compilation and execution of a program. Even though the code seems error free, the output generated is different from the expected one. These types of errors are called Logical Errors. Logical errors are those errors in which we think that our code is correct, the code compiles without any error and gives no error while it is running, but the output we get is different from the output we expected.
In 1999, NASA lost a spacecraft due to a logical error. This happened because of some miscalculations between the English and the American Units. The software was coded to work for one system but was used with the other.
For Example:
#include <stdio.h>

void main() {
    float a = 10;
    float b = 5;
    
    if (b = 0) {  // we wrote = instead of ==
        printf("Division by zero is not possible");
    } else {
        printf("The output is: %f", a/b);
    }
}


Output:

INF signifies a division by zero error. In the above example, at line 8, we wanted to check whether the variable b was equal to zero. But instead of using the equal to comparison operator (==), we use the assignment operator (=). Because of this, the if statement became false and the value of b became 0. Finally, the else clause got executed.
4. Semantic Error
Errors that occur because the compiler is unable to understand the written code are called Semantic Errors. A semantic error will be generated if the code makes no sense to the compiler, even though it is syntactically correct. It is like using the wrong word in the wrong place in the English language. For example, adding a string to an integer will generate a semantic error.
Semantic errors are different from syntax errors, as syntax errors signify that the structure of a program is incorrect without considering its meaning. On the other hand, semantic errors signify the incorrect implementation of a program by considering the meaning of the program.
The most commonly occurring semantic errors are: use of un-initialized variables, type compatibility, and array index out of bounds.



  


                       
  




Example 1:
#include <stdio.h>

void main() {
    int a, b, c;
    
    a * b = c;
    // This will generate a semantic error
}


Output:
error: lvalue required as left operand of assignment


When we have an expression on the left-hand side of an assignment operator (=), the program generates a semantic error. Even though the code is syntactically correct, the compiler does not understand the code.
Example 2:
#include <stdio.h>

void main() {
    int arr[5] = {5, 10, 15, 20, 25};
    
    int arraySize = sizeof(arr)/sizeof(arr[0]);
    
    for (int i = 0; i <= arraySize; i++)
    {
        printf("%d n", arr[i]);
    }
}


Output:

In the above example, we printed six elements while the array arr only had five. Because we tried to access the sixth element of the array, we got a semantic error and hence, the program generated a garbage value.
5. Linker Error
Linker is a program that takes the object files generated by the compiler and combines them into a single executable file. Linker errors are the errors encountered when the executable file of the code can not be generated even though the code gets compiled successfully. This error is generated when a different object file is unable to link with the main object file. We can run into a linked error if we have imported an incorrect header file in the code, we have a wrong function declaration, etc.
For Example:
#include <stdio.h>
 
void Main() { 
    int var = 10;
    printf("%d", var);
}


Output:
undefined reference to `main'


In the above code, as we wrote Main() instead of main(), the program generated a linker error. This happens because every file in the C language must have a main() function. As in the above program, we did not have a main() function, the program was unable to run the code, and we got an error. This is one of the most common type of linker error.

Conclusion

There are 5 different types of errors in C programming language: Syntax error, Runtime error, Logical error, Semantic error, and Linker error.
Syntax errors, linker errors, and semantic errors can be identified by the compiler during compilation. Logical errors and run time errors are encountered after the program is compiled and executed.
Syntax errors, linker errors, and semantic errors are relatively easy to identify and rectify compared to the logical and run time errors. This is so because the compiler generates these 3 (syntax, linker, semantic) errors during compilation itself, while the other 2 errors are generated during or after the execution.

Очень часто начинающие программисты впадают в суеверный ужас, когда видят, что компилятор нашел в тексте программы ошибку, но не понимают, в чем она заключается.

А если помножить этот факт на незнание английского языка («чего там ему не нравится?..») и слабое владение синтаксисом C++ («хм, а может, тут нужна точка с запятой…»), то проблема принимает масштаб катастрофы.

Тот факт, что компилятор в силу своих ограниченных возможностей изо всех сил старается объяснить, что конкретно неверно, не спасает ситуацию. Как быть, если гуглить неохота, а спросить не у кого?

В этом посте на правах копипаста с последующим переводом, дополнениями и исправлениями приведу описание наиболее распространенных сообщений об ошибках и предупреждений компилятора. Неприятность кроется в том факте, что разные компиляторы ругаются на одинаковые ошибки по-разному, а некоторые даже не замечают то, что другие принимают за ошибку. Все зависит от совести разработчиков компилятора, даты его выпуска, и др.

В качестве компилятора возьмем g++, который, в частности, может использоваться в среде Code::Blocks. Версия gcc (куда входит g++) для ОС Windows зовется MinGW. По ходу я буду давать аналоги ошибок из лексикона русскоязычной Microsoft Visual C++.

Итак, частые ошибки:

undeclared identifier

1) Пример

doy.cpp: In function 'int main()':
doy.cpp:25: 'DayOfYear' undeclared (first use this function)
doy.cpp:25: (Each undeclared identifier is reported only once for each function it appears in.)
doy.cpp:25: parse error before ';' token

2) Смысл
Использован идентификатор DayOfYear, но компилятор не нашел его объявления. Он не знает, что такое DayOfYear.

3) Когда бывает

Вы забыли включить какой-то заголовочный файл (#include...)
Вы где-то ошиблись в написании идентификатора (при объявлении или использовании)
Вы вообще забыли, что эту переменную надо объявить

Попытавшись скомпилировать это в Microsoft Visual C++, вы увидите:

error C2065: DayOfYear: необъявленный идентификатор

cout undeclared

1) Пример

xyz.cpp: In function 'int main()':
xyz.cpp:6: 'cout' undeclared (first use this function)
xyz.cpp:6: (Each undeclared identifier is reported only once for each function it appears in.)

2) Смысл
Суперклассика. Без комментариев.

3) Когда бывает

Вы забыли включить <iostream>
Вы забыли написать using namespace std;

jump to case label

1) Пример

switch.cpp: In function 'int main()':
switch.cpp:14: jump to case label
switch.cpp:11: crosses initialization of 'int y'

2) Смысл
Смысл туманен

3) Когда бывает
Вы попытались объявить и инициализировать переменную (объект, указатель и т.п.) в метке case оператора выбора switch. Правилами C++ это запрещено.

В Microsoft Visual C++ эта ошибка зовется

error C2360: пропуск инициализации 'y' из-за метки 'case'

Выход: заключите операторы этого case’а в фигурные скобки {}.

multi-line string / unterminated string

1) Пример
Программка

#include <iostream>

using namespace std;

int main()
{
cout << "Bob is my buddy;
cout << "and so is Mary" << endl;
}

вызовет бурную реакцию компилятора:

string.cpp:7:12: warning: multi-line string literals are deprecated
string.cpp: In function 'int main()':
string.cpp:7: 'so' undeclared (first use this function)
string.cpp:7: (Each undeclared identifier is reported only once for each function it appears in.)
string.cpp:7: parse error before 'Mary'
string.cpp:8:28: warning: multi-line string literals are deprecated
string.cpp:8:28: missing terminating " character
string.cpp:7:12: possible start of unterminated string literal

2) Смысл
Компилятор думает, что мы хотим создать строковую константу с содержащимся в ней переносом строки, что-то типа

"Hello
world!"

что не поддерживается языком. Также делается предположение о том, что мы, возможно, забыли поставить кавычки в конце первой строки. Собственно, так оно и есть.

3) Когда бывает
Когда не соблюдается правильное количество и положение кавычек в строковых литералах. Надо быть внимательнее.

Microsoft Visual C++ со свойственной ему детской непосредственностью, отметит, что нельзя делать переносы в строках и возмутится, где точка с запятой:

error C2001: newline в константе
error C2146: синтаксическая ошибка: отсутствие ";" перед идентификатором "cout"

comparison between signed and unsigned integer expressions

1) Пример

xyz.cpp: In function 'int main()':
xyz.cpp:54: warning: comparison between signed and unsigned integer expressions

2) Смысл
Это — предупреждение компилятора, которое говорит о том, что мы пытаемся сравнить (==, и т.д.) целочисленное выражение (может принимать положительные, отрицательные значения и 0) и беззнаковое целочисленное выражение (может быть только положительным, либо 0).

3) Когда бывает
Собственно, тогда и бывает. Напомню, что тип int по умолчанию знаковый, а некоторые функции (например, vector::size()) возвращают unsigned int.
К примеру, следующий на первый взгляд безобидный код вызовет описываемое предупреждение:

for (int i = 0; i < grades.size(); i++)
{
// ...
}

Следует помнить, что в памяти знаковые и беззнаковые типы имеют разные внутренние представления, поэтому надо быть чертовски осторожными с указателями.

В Microsoft Visual C++ предупреждение выглядит так:

warning C4018: <: несоответствие типов со знаком и без знака

suggest parentheses around assignment used as truth value

1) Пример

xyz.cpp: In function `int main()':
xyz.cpp:54: warning: suggest parentheses around assignment used as truth value

2) Смысл
Тоже классика. Компилятор предполагает (и в 99% случаев прав), что вы по ошибке включили в скобки в качестве условия для if/while/for вместо условного выражения выражение присваивания.

3) Когда бывает
Чаще всего — в if‘ах, когда вместо "==" используется "="

if (length = maxLength)

вместо

if (length == maxLength)

Заминка в том, что это не ошибка, т.к. в скомпилированной программе (если мы проигнорируем предупреждение) выражение присваивания (которое возвращает значение правого аргумента) во всех случаях, кроме тех, когда оно вернет 0, будет преобразовано к true.

Ссылки для дальнейшего изучения
Ошибки построения Microsoft Visual C++
GCC Compiler error messages
GCC Warnings

P.S. Следует отметить, что кроме ошибок стадии компиляции встречаются (гораздо реже) ошибки препроцессора (например, если не найден заголовочный файл <iostram>), ошибки стадии компоновки (можно избежать, если научиться пользоваться средой программирования) и — самый гнусный тип ошибок! — ошибки стадии выполнения. Т.н. runtime error. С ними может справиться только голова программиста, вооруженная отладчиком.

Источник

Variables: A variable is the name given to a memory location. It is the basic unit of storage in a program.

The value stored in a variable can be changed during program execution.
A variable is only a name given to a memory location, all the operations done on the variable effects that memory location.
All the variables must be declared before use.

How to declare variables?
We can declare variables in common languages (like C, C++, Java etc) as follows:

where:
datatype: Type of data that can be stored in this variable.
variable_name: Name given to the variable.
value: It is the initial value stored in the variable.

How to avoid errors while creating variables?

The identifier is undeclared: In any programming language, all variables have to be declared before they are used. If you try to use the name of a such that hasn’t been declared yet, an “undeclared identifier” compile-error will occur.
Example:

#include <stdio.h>

int main()

{

printf("%d", x);

return 0;

}

Compile Errors:
```
prog.c: In function 'main':
prog.c:5:18: error: 'x' undeclared (first use in this function)
     printf("%d", x);
                  ^
prog.c:5:18: note: each undeclared identifier is reported
     only once for each function it appears in
```
No initial value is given to the variable: This error commonly occurs when the variable is declared, but not initialized. It means that the variable is created but no value is given to it. Hence it will take the default value then. But in C language, this might lead to error as this variable can have a garbage value (or 0) as its default value. In other languages, 0 will be its default value.
Example:

#include <stdio.h>

int main()

{

    int x;

    printf("%d", x);

    return 0;

}

Output:
```
0
```
Using variable out of its Scope: Scope of a variable is the part of the program where the variable is accessible. Like C/C++, in Java, all identifiers are lexically (or statically) scoped, i.e.scope of a variable can be determined at compile time and independent of the function call stack.
Example:

#include <stdio.h>

int main()

{

    {

        int x = 5;

    }

    printf("%d", x);

    return 0;

}

Compile Errors:
```
prog.c: In function 'main':
prog.c:5:18: error: 'x' undeclared (first use in this function)
     printf("%d", x);
                  ^
prog.c:5:18: note: each undeclared identifier is reported
     only once for each function it appears in
```
How to Correct the above code: Declare the variable x before using it in the outer scope. Or you can use the already defined variable x in its own scope

Example:

#include <stdio.h>

int main()

{

    {

        int x = 5;

        printf("%d", x);

    }

    return 0;

}

Output:
```
5
```
Creating a variable with an incorrect type of value: This arises due to the fact that values are implicitly or explicitly converted into another type. Sometimes this can lead to Warnings or errors.
Example:

#include <stdio.h>

int main()

{

    char* x;

    int i = x;

    printf("%d", x);

    return 0;

}

Warning:
```
prog.c: In function 'main':
prog.c:7:13: warning: initialization makes integer
 from pointer without a cast [-Wint-conversion]
     int i = x;
             ^
```

Last Updated :
05 Apr, 2019

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Итак, частые ошибки:

undeclared identifier

1) Пример

3) Когда бывает

Вы забыли включить какой-то заголовочный файл (#include...)
Вы где-то ошиблись в написании идентификатора (при объявлении или использовании)
Вы вообще забыли, что эту переменную надо объявить

Попытавшись скомпилировать это в Microsoft Visual C++, вы увидите:

error C2065: DayOfYear: необъявленный идентификатор

cout undeclared

1) Пример

xyz.cpp: In function 'int main()':
xyz.cpp:6: 'cout' undeclared (first use this function)
xyz.cpp:6: (Each undeclared identifier is reported only once for each function it appears in.)

2) Смысл
Суперклассика. Без комментариев.

3) Когда бывает

Вы забыли включить <iostream>
Вы забыли написать using namespace std;

jump to case label

1) Пример

switch.cpp: In function 'int main()':
switch.cpp:14: jump to case label
switch.cpp:11: crosses initialization of 'int y'

2) Смысл
Смысл туманен

В Microsoft Visual C++ эта ошибка зовется

error C2360: пропуск инициализации 'y' из-за метки 'case'

Выход: заключите операторы этого case’а в фигурные скобки {}.

multi-line string / unterminated string

1) Пример
Программка

#include <iostream>

using namespace std;

int main()
{
cout << "Bob is my buddy;
cout << "and so is Mary" << endl;
}

вызовет бурную реакцию компилятора:

"Hello
world!"

error C2001: newline в константе
error C2146: синтаксическая ошибка: отсутствие ";" перед идентификатором "cout"

comparison between signed and unsigned integer expressions

1) Пример

xyz.cpp: In function 'int main()':
xyz.cpp:54: warning: comparison between signed and unsigned integer expressions

for (int i = 0; i < grades.size(); i++)
{
// ...
}

В Microsoft Visual C++ предупреждение выглядит так:

warning C4018: <: несоответствие типов со знаком и без знака

suggest parentheses around assignment used as truth value

1) Пример

xyz.cpp: In function `int main()':
xyz.cpp:54: warning: suggest parentheses around assignment used as truth value

3) Когда бывает
Чаще всего — в if‘ах, когда вместо "==" используется "="

if (length = maxLength)

вместо

if (length == maxLength)

Ссылки для дальнейшего изучения
Ошибки построения Microsoft Visual C++
GCC Compiler error messages
GCC Warnings

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