C++ Error Reference

Error Reference

This lesson serves as a comprehensive reference for common C++ errors you'll encounter when working with variables, data types, operators, expressions, and input/output. Use this as a quick lookup guide when you encounter unfamiliar error messages.

Variable Declaration and Initialization Errors

1. Using Uninitialized Variables

Problematic code:

#include <iostream>

int main() {
    int count;
    std::cout << count + 5 << '\n';  // Using uninitialized variable
    return 0;
}

Error message (varies by compiler):

warning: variable 'count' is uninitialized when used here

Why this happens: Local variables aren't automatically initialized to zero - they contain garbage values.

Fix: Always initialize variables when you declare them:

int count{};  // Modern initialization - defaults to 0
// or
int count = 0;  // Traditional initialization

2. Variable Name Conflicts

Problematic code:

#include <iostream>

int main() {
    int age{25};
    int age{30};  // Redeclaring the same variable
    return 0;
}

Error message:

error: redeclaration of 'int age'
note: 'int age' previously declared here

Fix: Use different variable names or update the existing variable:

int age{25};
age = 30;  // Assignment, not declaration

3. Invalid Variable Names

Problematic code:

#include <iostream>

int main() {
    int 2users = 10;  // Can't start with number
    int user-name = 5;  // Hyphens not allowed
    int class = 8;  // 'class' is a reserved keyword
    return 0;
}

Error messages:

error: expected primary-expression before 'int'

Note: This is the error you get when using a reserved keyword like 'class'. The actual error for invalid variable names starting with numbers or containing hyphens may be similar.

Fix: Follow variable naming rules:

int numUsers{10};     // Start with letter
int userName{5};      // Use camelCase or underscores
int studentClass{8};  // Avoid reserved keywords

Data Type Errors

4. Type Mismatch in Assignment

Problematic code:

#include <iostream>

int main() {
    int number = "Hello";        // String to int
    char letter = 65.5;         // Double to char
    bool flag = "true";         // String to bool
    return 0;
}

Error message:

error: invalid conversion from 'const char*' to 'int' [-fpermissive]

Note: This example shows the error for just the string-to-int conversion. Similar errors occur for the other type mismatches.

Fix: Match types correctly:

int number{42};           // int to int
char letter{'A'};         // char literal
bool flag{true};          // bool literal

5. Integer Overflow Warnings

Problematic code:

#include <iostream>

int main() {
    short smallNumber = 50000;  // Too big for short
    char grade = 300;           // Too big for char
    return 0;
}

Error messages:

warning: implicit conversion from 'int' to 'short' changes value from 50000 to -15536
warning: implicit conversion changes signedness

Fix: Use appropriate data types:

int largeNumber{50000};   // Use int for larger values
char grade{'A'};          // Use char for characters, not large numbers

Expression and Operator Errors

6. Missing Operators

Problematic code:

#include <iostream>

int main() {
    int a{5};
    int b{3};
    int result = a b;  // Missing operator between variables
    return 0;
}

Error message:

error: expected ';' before 'b'

Fix: Add the missing operator:

int result{a + b};  // Addition
int result{a * b};  // Multiplication

7. Invalid Operations on Different Types

Problematic code:

#include <iostream>

int main() {
    std::string name = "Alice";
    int age = 25;
    std::string result = name + age;  // Can't add string + int directly
    return 0;
}

Error message:

error: no operator "+" matches these operands

Fix: Convert types appropriately:

std::string result{name + std::to_string(age)};  // Convert int to string first

8. Division by Zero (Runtime Warning)

Problematic code:

#include <iostream>

int main() {
    int a{10};
    int b{0};
    int result{a / b};  // Division by zero
    std::cout << result << '\n';
    return 0;
}

Note: This compiles but causes undefined behavior at runtime.

Fix: Check for zero before dividing:

if (b != 0) {
    int result{a / b};
    std::cout << result << '\n';
} else {
    std::cout << "Error: Division by zero!" << '\n';
}

Input/Output (iostream) Errors

9. Missing std:: Prefix

Problematic code:

#include <iostream>

int main() {
    cout << "Hello World" << endl;  // Missing std:: prefix
    return 0;
}

Error message:

error: use of undeclared identifier 'cout'
error: use of undeclared identifier 'endl'

Fix: Add the std:: prefix:

std::cout << "Hello World" << '\n';

10. Wrong Stream Direction

Problematic code:

#include <iostream>

int main() {
    int age;
    std::cout >> "Enter age: ";     // Wrong direction for output
    std::cin << age;                // Wrong direction for input
    return 0;
}

Error message:

error: no match for 'operator>>' (operand types are 'std::ostream' and 'const char [12]')

Note: The actual error message is quite long and technical, but the key part is "no match for 'operator>>'" which tells you you're using the wrong operator direction.

Fix: Use correct stream directions:

std::cout << "Enter age: ";  // << for output
std::cin >> age;             // >> for input

11. Mismatched Quotes

Problematic code:

#include <iostream>

int main() {
    std::cout << "Hello World';     // Mixed quote types
    std::cout << 'Hello World";     // Wrong quote for string
    return 0;
}

Error messages:

error: missing terminating " character
warning: multi-character character constant

Fix: Use consistent quote types:

std::cout << "Hello World";   // Double quotes for strings
std::cout << 'A';             // Single quotes for single characters

Expression Statement Errors

12. Unused Expression Results

Problematic code:

#include <iostream>

int main() {
    int a{5};
    int b{3};
    a + b;  // Expression result is unused
    return 0;
}

Warning message:

warning: expression result unused

Fix: Use the result or make the intention clear:

int result{a + b};           // Store the result
std::cout << a + b << '\n';  // Use the result directly

Initialization Errors

13. Uniform Initialization Issues

Problematic code:

#include <iostream>

int main() {
    int x{3.14};  // Narrowing conversion not allowed with {}
    return 0;
}

Error message:

error: type 'double' cannot be narrowed to 'int' in initializer list

Fix: Use compatible types:

int x{3};        // Use integer literal
double x{3.14};  // Use double variable

When you intentionally want to convert a type, you can use static_cast to make the conversion explicit:

int x{static_cast<int>(3.14)}; // Explicit conversion

Conditional Logic Errors

14. Assignment vs Comparison in if Statements

Problematic code:

#include <iostream>

int main() {
    int age{18};
    if (age = 21) {  // Assignment instead of comparison
        std::cout << "Can drink" << '\n';
    }
    return 0;
}

Warning message:

warning: suggest parentheses around assignment used as truth value

Fix: Use comparison operator:

if (age == 21) {  // Comparison
    std::cout << "Can drink" << '\n';
}

15. Missing Braces in if Statements

Problematic code:

#include <iostream>

int main() {
    int score{85};
    if (score > 90)
        std::cout << "Grade: A" << '\n';
        std::cout << "Excellent work!" << '\n';  // Always executes!
    return 0;
}

This compiles but doesn't work as expected.

Fix: Always use braces:

if (score > 90) {
    std::cout << "Grade: A" << '\n';
    std::cout << "Excellent work!" << '\n';
}