Related
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)
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.
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.
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.
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.
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
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
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.
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
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.
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.
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.
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.
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
I am in the process of creating a C wrapper around a C++ library.
One common mistake to make while doing this is having a function declaration and definition that do not match for some reason (typo, renames, argument got added/removed, etc).
For example:
// enabledata.h
MDS_C_API const char* motek_mds_enable_data_get_enable_command_name();
// enabledata.cpp
const char* motek_mds_enable_data_enable_command_name() { ... }
The names do not match, but because of the lack of scope for these functions, it will not result in any compile errors, and will only show up much later down the line as a link error.
I want the compiler to help me find these errors by using the global scope operator like so:
const char* ::motek_mds_enable_data_get_disable_command_name() { ... }
This will now show up as a compile error if the function has not been declared yet, which is exactly what I want.
However, this does not work when the function returns a typedef:
int32_t ::motek_mds_enable_data_is_enabled(const Data* a_Data) { ... }
This will result in an attempt to use int32_t as a scope, which of course results in an error:
left of '::' must be a class/struct/union
Are there any ways to make this work? Better alternatives are also welcome of course.
I am currently using Visual Studio 2015 Update 2.
You can always parenthesize the declarator-id:
int32_t (::motek_mds_enable_data_is_enabled)(const Data* a_Data) { ... }
// ^ ^
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)
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.
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.
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.
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.
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
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
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.
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
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.
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.
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.
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.
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
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)
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.
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.
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.
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.
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
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
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.
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
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.
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.
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.
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.
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
I am just trying to check whether compiler allows type name as variable name.
When i tried
int int;
It reported an error saying
error C2632: 'int' followed by 'int' is illegal
But when i tried
#include <string>
using namespace std;
int main()
{
string string;
}
It didn't give any error.
Both string and int are data types.
Why compiler allows string and doesn't allow int ?
EDIT: includes updated.
EDIT: Some people are saying that int is not a class.
In that case, why below line is allowed.
int a(10);
it works similar to constructor of a class.
string is not a C++ reserved word, but int is, and a reserved word cannot be used as an identifier.
And its syntactically fine to have class name and object name to be same.
class test {};
int main() {
test test; // test is an object of type test.
}
int is a C++ keyword. In the second declaration 'string string' declares an object of type 'std::string'. After this the name 'string' hides 'std::string' in an unqualified lookup
#include <string>
using std::string;
int main(){
string string;
string s; // Error
}
int is a keyword, whereas string is the name of a class in the standard library but is not a keyword.
string isn't actually a "data type" in the same sense the int is. int is a "native type" and as such, the text "int" is a keyword.
string is just a "user-defined" class (although, here the "user" the defined it is the C++ standards committtee). But as such, "string" is not a keyword.
So int int is two keywords together, but string string is just defining a varaible named "string" of type "string". The C++ compiler can keeps the separate two uses of "string" straight (but it's not a good idea to do this, since, as you've demonstrated, programmers often can't).
Well, given that others have essentially posted the answer, I'm going to go ahead and post what I meant...
I'm assuming that because the second answer compiles, that you have using namespace std; in your file (which is in general not a good idea; I fail to see why people tell beginning C++ users to do this).
When the compiler goes to resolve the first string, it is able to find the class in namespace std. The second use of string is simply the name of the variable.
The compiler allows primitive types to behave like classes for the purposes of templates - if you had to specialize for primitives everywhere it would be a nightmare.
Try removing
#include <string>