Quick Answer

Caused by missing implementations or linking issues.

Understanding the Issue

This error occurs when declarations exist but implementations are missing or not properly linked. Common with templates and static libraries.

The Problem

This code demonstrates the issue:

Cpp Error 
// Problem: Declaration without definition
void helper(); // Declaration
int main() {
    helper(); // Linker error

The Solution

Here's the corrected code:

Cpp Fixed 
// Solution: Provide implementation
void helper() { /*...*/ }

// For templates: ensure definitions are visible

Key Takeaways

Verify all declared functions have implementations.