C++: error: 'x' was not declared in this scope (undeclared identifier)

Meaning

C++ reads a file top to bottom, so a name must be declared before it is used. The usual causes are a missing #include, a missing std:: prefix, a typo (case matters), or using a variable outside the block where it was declared. GCC's message ends with a "suggested alternative" when it finds a close match.

Common causes

• Missing #include for the function or type (for example <vector>, <cstring>)
• Missing std:: or using namespace std
• Typo or wrong capitalisation in the name
• Variable declared inside a block and used outside it
• Function used before its declaration or prototype

Detailed fix by platform

Code examples

#include <iostream>

int main() {
    vector<int> v{1, 2, 3};          // error: 'vector' was not declared in this scope
    cout << v.size() << "\n";         // error: 'cout' was not declared in this scope
}

// Fix:
// #include <vector>
// std::vector<int> v{1, 2, 3};
// std::cout << v.size() << "\n";

How to diagnose

1. Name — Spelling and case exactly right?
2. Header — Is the declaring header included in THIS file?
3. Scope — Is the name used inside the block where it was declared?

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