What is an 'undeclared identifier' error and how do I fix it?

Question

What are undeclared identifier errors? What are common causes and how do I fix them?

Example error texts:

• For the Visual Studio compiler: error C2065: 'cout' : undeclared identifier
• For the GCC compiler: 'cout' undeclared (first use in this function)

Answer 1

They most often come from forgetting to include the header file that contains the function declaration, for example, this program will give an 'undeclared identifier' error:

Missing header

int main() {     std::cout << "Hello world!" << std::endl;     return 0; } 

To fix it, we must include the header:

#include <iostream> int main() {     std::cout << "Hello world!" << std::endl;     return 0; } 

If you wrote the header and included it correctly, the header may contain the wrong include guard.

To read more, see http://msdn.microsoft.com/en-us/library/aa229215(v=vs.60).aspx.

Misspelled variable

Another common source of beginner's error occur when you misspelled a variable:

int main() {     int aComplicatedName;     AComplicatedName = 1;  /* mind the uppercase A */     return 0; } 

Incorrect scope

For example, this code would give an error, because you need to use std::string:

#include <string>  int main() {     std::string s1 = "Hello"; // Correct.     string s2 = "world"; // WRONG - would give error. } 

Use before declaration

void f() { g(); } void g() { } 

g has not been declared before its first use. To fix it, either move the definition of g before f:

void g() { } void f() { g(); } 

Or add a declaration of g before f:

void g(); // declaration void f() { g(); } void g() { } // definition 

stdafx.h not on top (VS-specific)

This is Visual Studio-specific. In VS, you need to add #include "stdafx.h" before any code. Code before it is ignored by the compiler, so if you have this:

#include <iostream> #include "stdafx.h" 

The #include <iostream> would be ignored. You need to move it below:

#include "stdafx.h" #include <iostream> 

_{Feel free to edit this answer.}