Quick Answer

Missing function implementations or linking problems

Understanding the Issue

This error occurs during linking when a declared function is called but no implementation can be found. Common causes include missing source files in compilation, incorrect function signatures, or forgetting to implement a function.

The Problem

This code demonstrates the issue:

Cpp Error 
// header.h
void helper();
// main.cpp
int main() {
    helper(); // Linker error
}

The Solution

Here's the corrected code:

Cpp Fixed 
// Solution 1: Add implementation
// helper.cpp
void helper() { /*...*/ }

// Solution 2: Check build system includes
// g++ main.cpp helper.cpp -o program

Key Takeaways

Verify all declared functions have implementations and are properly linked.