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)

Gayelord

A very common case is including <iostream> and trying to use std::string without having <string> included. May be worth mentioning. – Decemvir

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.}

Gayelord

people shouldn't edit others answers just correct it if it's wrong or make it look better ... , but feel free to add answer contain something else and you are free to include them to your answer and mention that this part from xxx – Thermobarometer

I just wasted a couple of hours looking at a similar problem and in my case it was to do with header guards not being correct. – Ashleighashlen

One other tricky case that leads to this error : https://mcmap.net/q/281215/-templates-parent-class-member-variables-not-visible-in-inherited-class – Ersatz

Another thing to check for are crossed references – Spry

Answer 2

Consider a similar situation in conversation. Imagine your friend says to you, "Bob is coming over for dinner," and you have no idea who Bob is. You're going to be confused, right? Your friend should have said, "I have a work colleague called Bob. Bob is coming over for dinner." Now Bob has been declared and you know who your friend is talking about.

The compiler emits an 'undeclared identifier' error when you have attempted to use some identifier (what would be the name of a function, variable, class, etc.) and the compiler has not seen a declaration for it. That is, the compiler has no idea what you are referring to because it hasn't seen it before.

If you get such an error in C or C++, it means that you haven't told the compiler about the thing you are trying to use. Declarations are often found in header files, so it likely means that you haven't included the appropriate header. Of course, it may be that you just haven't remembered to declare the entity at all.

Some compilers give more specific errors depending on the context. For example, attempting to compile X x; where the type X has not been declared with clang will tell you "unknown type name X". This is much more useful because you know it's trying to interpret X as a type. However, if you have int x = y;, where y is not yet declared, it will tell you "use of undeclared identifier y" because there is some ambiguity about what exactly y might represent.

Semiskilled

Answer 3

In C and C++ all names have to be declared before they are used. If you try to use the name of a variable or a function that hasn't been declared you will get an "undeclared identifier" error.

However, functions are a special case in C (and in C only) in that you don't have to declare them first. The C compiler will the assume the function exists with the number and type of arguments as in the call. If the actual function definition does not match that you will get another error. This special case for functions does not exist in C++.

You fix these kind of errors by making sure that functions and variables are declared before they are used. In the case of printf you need to include the header file <stdio.h> (or <cstdio> in C++).

For standard functions, I recommend you check e.g. this reference site, and search for the functions you want to use. The documentation for each function tells you what header file you need.

Autunite

Answer 4

I had the same problem with a custom class, which was defined in a namespace. I tried to use the class without the namespace, causing the compiler error "identifier "MyClass" is undefined". Adding

using namespace <MyNamespace>

or using the class like

MyNamespace::MyClass myClass;

solved the problem.

Mallina

Answer 5

These error meassages

1.For the Visual Studio compiler: error C2065: 'printf' : undeclared identifier
2.For the GCC compiler: `printf' undeclared (first use in this function)

mean that you use name printf but the compiler does not see where the name was declared and accordingly does not know what it means.

Any name used in a program shall be declared before its using. The compiler has to know what the name denotes.

In this particular case the compiler does not see the declaration of name printf . As we know (but not the compiler) it is the name of standard C function declared in header <stdio.h> in C or in header <cstdio> in C++ and placed in standard (std::) and global (::) (not necessarily) name spaces.

So before using this function we have to provide its name declaration to the compiler by including corresponding headers.

For example C:

#include <stdio.h>

int main( void )
{
   printf( "Hello World\n" );
}

C++:

#include <cstdio>

int main()
{
   std::printf( "Hello World\n" );
   // or printf( "Hello World\n" );
   // or ::printf( "Hello World\n" );
}

Sometimes the reason of such an error is a simple typo. For example let's assume that you defined function PrintHello

void PrintHello()
{
    std::printf( "Hello World\n" );
}

but in main you made a typo and instead of PrintHello you typed printHello with lower case letter 'p'.

#include <cstdio>

void PrintHello()
{
    std::printf( "Hello World\n" );
}

int main()
{
   printHello();
}

In this case the compiler will issue such an error because it does not see the declaration of name printHello. PrintHello and printHello are two different names one of which was declared and other was not declared but used in the body of main

Gorget

In C++ the versions without std:: are not guaranteed to work – Bedpost

Answer 6

It happened to me when the auto formatter in a visual studio project sorted my includes after which the pre compiled header was not the first include anymore.

In other words. If you have any of these:

#include "pch.h"

or

#include <stdio.h>

or

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

Put it at the start of your file.

If your clang formatter is sorting the files automatically, try putting an enter after the pre compiled header. If it is on IBS_Preserve it will sort each #include block separately.

#include "pch.h"  // must be first

#include "bar.h"  // next block
#include "baz.h"
#include "foo.h"

More info at Compiler Error C2065

Overarch

If you try to create a dynamic link library project in Visual Studio, you must put the #include "pch.h" at the first line. Or the C2065 error will be reported. – Jacquejacquelin

Answer 7

A C++ identifier is a name used to identify a variable, function, class, module, or any other user-defined item. In C++ all names have to be declared before they are used. If you try to use the name of a such that hasn't been declared you will get an "undeclared identifier" compile-error.

According to the documentation, the declaration of printf() is in cstdio i.e. you have to include it, before using the function.

Hedgehog

Answer 8

Another possible situation: accessing parent (a template class) member in a template class.

Fix method: using the parent class member by its full name (by prefixing this-> or parentClassName:: to the name of the member).

see: templates: parent class member variables not visible in inherited class

Mellon

Answer 9

one more case where this issue can occur,

if(a==b)
double c;
getValue(c);

here, the value is declared in a condition and then used outside it.

Bern

Answer 10

It is like Using the function without declaring it. header file will contain the function printf(). Include the header file in your program this is the solution for that. Some user defined functions may also through error when not declared before using it. If it is used globally no probs.

Uncanonical

Answer 11

Every undeclared variable in c error comes because the compiler is not able to find it in the project. One can include the external (header) file of the library in which the variable is defined. Hence in your question, you require <stdio.h>, that is a standard input output file, which describes printf(), functionality.

According to the documentation, the declaration of fprintf() is in i.e. you have to include it, before using the function.

Unnatural

By mistake, I edited your answer. My intention was to edit mine. – Hedgehog

Answer 12

Most of the time, if you are very sure you imported the library in question, Visual Studio will guide you with IntelliSense.

Here is what worked for me:

Make sure that #include "stdafx.h" is declared first, that is, at the top of all of your includes.

Oasis

Answer 13

Check if you are importing the same packages in your .m and in your .h Example given: I had this very problem with the init method and it was caused by missing the "#import " on the .m file

Mcmullen