I have the following C++ test program called test.cpp:
#include <cmath>
#include <iostream>
double sqrt(double d) { return std::sqrt(d); }
int main()
{
std::cout << "sqrt(4): " << sqrt(4) << std::endl;
}
This is some pretty contrived code, and as you might have guessed I'm just trying to do an exercise out of Stroustrup. He declared double sqrt(double), and wants the reader to define it.
I compiled the above code using g++ 4.8 (from the MINGW release of Qt 5.1):
C:\Windows\Temp>g++ -o test.exe -g test.cpp
When I ran the resulting executable, Windows 7 said "test.exe has stopped working".
To see what went wrong, I ran test.exe in the GNU debugger. Debugger commands and output:
C:\Windows\Temp>gdb -q test.exe
Reading symbols from C:\Windows\Temp\test.exe...done.
(gdb) b main
Breakpoint 1 at 0x401625: file test.cpp, line 8.
(gdb) run
Starting program: C:\Windows\Temp\test.exe
[New Thread 12080.0x2ba0]
Breakpoint 1, main () at test.cpp:8
8 std::cout << "sqrt(4): " << sqrt(4) << std::endl;
(gdb) s
sqrt (d=4) at test.cpp:4
4 double sqrt(double d) { return std::sqrt(d); }
(gdb) s
sqrt (d=4) at test.cpp:4
4 double sqrt(double d) { return std::sqrt(d); }
(gdb) s
sqrt (d=4) at test.cpp:4
4 double sqrt(double d) { return std::sqrt(d); }
(gdb) s
sqrt (d=4) at test.cpp:4
4 double sqrt(double d) { return std::sqrt(d); }
(gdb) q
A debugging session is active.
Inferior 1 [process 12080] will be killed.
Quit anyway? (y or n) y
C:\Windows\Temp>
From the behavior and warning, I infer that std::sqrt must be calling sqrt from the global namespace -- which causes my function to be repeatedly invoked.
It would be easy enough to work around the unwanted recursion by changing the name of my sqrt function, or by putting it inside a namespace. But I would like to understand why std::sqrt is implemented in such a way that ::sqrt is called. I thought the whole point of the std namespace was to prevent name clashes with unqualified names in user code.
I took a peek at the source code for the GNU implementation of <cmath>. However, I lost the trail after following a few #includes in the chain. Maybe you can make more sense of it:
00052 #include <math.h>
00053
00054 // Get rid of those macros defined in <math.h> in lieu of real functions.
....
00076 #undef sqrt
....
00081 namespace std
00082 {
....
00393 using ::sqrt;
00394
00395 inline float
00396 sqrt(float __x)
00397 { return __builtin_sqrtf(__x); }
00398
00399 inline long double
00400 sqrt(long double __x)
00401 { return __builtin_sqrtl(__x); }
00402
00403 template<typename _Tp>
00404 inline typename __enable_if<double, __is_integer<_Tp>::_M_type>::_M_type
00405 sqrt(_Tp __x)
00406 { return __builtin_sqrt(__x); }
....
00437 }
Incidentally, this is not just a GNU puzzle. Compiling with the Visual C++ compiler instead of g++ yields the following warning:
C:\Windows\Temp>cl /nologo /EHsc test.cpp
test.cpp
c:\windows\temp\test.cpp(4) : warning C4717: 'sqrt' : recursive on all control
paths, function will cause runtime stack overflow
I guess that makes this a fair question to ask on StackOverflow. :)
Running the resulting executable leads to the expected outcome: "test.exe has stopped working".
The problem is that the functions inherited from the C standard library like, e.g., the <cmath> functions are somewhat funny beasts: they are made to look as if they live in namespace std but really they are extern "C" functions living in the global namespace. Basically calling std::sqrt(x) effectively calls ::sqrt(x) which happens to be the function you just defined!
I haven't checked what the C++ standard says about these names in the global namespace but I'd be fairly certain that it classifies them as reserved names. That is, you'd better not define ::sqrt in any shape or form. Define the function in a suitable namespace and you'll be fine.
OK, I checked. The relevant clause is 17.3.24 [defns.reserved.function]:
reserved function
a function, specified as part of the C++ standard library, that must be defined by the implementation [ Note: If a C++ program provides a definition for any reserved function, the results are undefined. —end note ]
... and 17.6.4.3.3 [extern.names] paragraph 3 and 4:
Each name from the Standard C library declared with external linkage is reserved to the implementation for use as a name with extern "C" linkage, both in namespace std and in the global namespace.
Each function signature from the Standard C library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage, or as a name of namespace scope in the global namespace.
17.6.4.3.3/2 Each global function signature declared with external linkage in a header is reserved to the implementation to designate that function signature with external linkage.
17.6.4.3.3/3 Each name from the Standard C library declared with external linkage is reserved to the implementation for use as a name with extern "C" linkage, both in namespace std and in the global namespace.
17.6.4.3.3/4 Each function signature from the Standard C library declared with external linkage is reserved to the implementation for use as a function signature with both extern "C" and extern "C++" linkage, or as a name of namespace scope in the global namespace.
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
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 have the following c++ program:
Client.h
#ifndef Client_Client_h
#define Client_Client_h
#include "Client.h"
class Client {
public:
void f1();
void f2();
};
#endif
Client.cpp
#include <iostream>
#include <stdlib.h>
using namespace std;
#include "Client.h"
void Client::f1(){
cout << "Client.f1()" << endl;
}
void Client::f2() {
cout << "Client.f2()" << endl;
}
compiling the above in XCode 4.3 gives me a static library file called:
libClient.a
Separately, I have a main.c
#include <stdio.h>
//
//using namespace std;
int main(){
// how do I do something like: Client c; c.f1(); c.f2();
// and actually get output ?
printf("hello\n");
return 0;
}
What's steps do I need to take in order to invoke f1() and f2() ? How do I use GCC to link the static library properly?
So far I have tried:
gcc -lClient.a main.c
which gives me :
ld: library not found for -lClient.a
collect2: ld returned 1 exit status
This isn't going to work, or at least is not going to be portable. The one really really obvious thing to do is to make your program C++ so you can access those features.
You can't "natively" use C++ code from C code, for obvious reasons. You don't have access to object-oriented features, so a ton of stuff isn't going to work: constructors, destructors, move/copy semantics and virtual inheritance are probably the biggest things that you'll miss. (That's right: you won't be able to create or destroy objects correctly, unless they have trivial constructors and destructors.)
You'll also run into linkage issues: C++ function names are mangled into a mess that includes their parameter types and return types and classes, which will look like __1cGstrcpy6Fpcpkc_0_. It would be technically feasible to declare the mangled names of the functions in C to use them, or use dlsym to get a pointer to them, but that's plain silly. Don't do that.
If you need to create a function in C++ that needs to be callable from C, you can specify it as extern "C" and its name won't be mangled, and it will be accessible from C, and it will be itself able to use C++ features:
extern "C" void Foo()
{
std::string hello = "Hello world!";
std::cout << hello << std::endl;
}
You will then need to declare it on the C side of your program like this:
void Foo();
And you'll be able to call it.
It's possible for you to wrap all your C++ calls that you want to expose to your C program in extern "C" functions, and return a pointer to your type and deal with it that way, but it's going to get annoying very quickly. You really should just use C++.
As far as linking with the static library is concerned, in Xcode, go to your project settings, pick your target, go to the Build Phases tab, unfold the "Link Binary With Libraries" section, and drop your .a file there.
A few days back, I came across this piece of C++ code, though I can't paste the code itself, I could recreate the problem with some sample code.
First, the file, namespace.h:
#include <iostream>
using namespace std;
namespace useless{
class X {
int m_myint;
static X *m_foobar;
X* getPrivVal(void);
public:
int getMember(void);
X* getStaticVal(void);
};
}
Next, namespace.cpp:
#include "namespace.h"
extern "C"{
namespace useless{
X* X::m_foobar = NULL;
X* X::getPrivVal(void){
if(m_foobar == NULL)
m_foobar = new X;
return(m_foobar);
}
}
}
namespace useless {
int X::getMember(void){
if(m_myint == 0)
m_myint = 1;
return(m_myint);
}
X* X::getStaticVal(void){
return(getPrivVal());
}
}
using namespace useless;
int main(void){
X y;
cout << "The int value is " << y.getMember() << endl;
cout << "The value of the static member is " << y.getStaticVal() << endl;
return(0);
}
This code compiled and linked fine when I used g++ 3.4.3, but gives the following error when I use g++ 4.x, I have tried with GCC 4.6.1 as well GCC 4.2.1. Have tried it on Linux as well as Mac, same results.
This is the error(Suse):
g++ -o namespace namespace.cpp
namespace.h:8:15: error: previous declaration of useless::X*
useless::X::m_foobar with C++ linkage
namespace.cpp:5:11: error: conflicts with new declaration with C linkage
Can someone please shed some light on what the legacy C++ code was looking to do, could this have been a hack or workaround to get around an old problem that I am not aware of any longer. Ohh by the way, the method inside the extern "C" is called by C++ code and not any C code as I initially suspected.
FYI, this is legacy code now, may have been written in 2001/2002 period.
Thanks guys. I have gone ahead and got rid of the extern "C" with no major impact.
#Bjorn Pollex : The guy who wrote that code is long gone.
The extern "C" directive has two effects, it disables mangling when possible and uses the C calling convention. In this particular case, because there is a namespace in between, name mangling cannot be disabled, so it could have been added to force a particular calling convention.
The code is actually wrong in that the declaration does not force extern "C" but the definition does, which is incorrect. Consider that if the compiler just followed the directive as shown, the caller would use C++ naming and calling conventions, while the function would be using the C variant, if the two differ the result will be undefined behavior.
Seems like an attempt to generate the variables witout C++ name mangling. That's one part of using extern "C".
The newer compiler obviously realizes that it really doesn't work.
If you have a piece of C++ code that you want to call from a module written in another language (C, FORTRAN or whatever), you want to turn off C++ name mangling.
Edit:
The really weird thing is that they did it on the definition but not on the declaration. It's a wonder that it ever compiled