Main无法调用函数：在此范围内未声明函数

Question

I am in the process of changing a C file to a C++ file (to eventually integrate it with a C program). I'm very new to C++ and in fact this is my first exposure to it. I have a test.cpp file that declares function main and hello as follows:

#include "test.h"

int main()
{
    hello ();
    return 0;
}

void hello()
{
    std::cout << "Hello there!" << endl;
}

The test.h file is declared as follows:

#include <iostream>

extern "C" void hello();

When I compile the program using g++ test.cpp, I get the error "hello was not declared in this scope".

Any suggestions?

Also, where does one find the API for C++ classes and their functions?

Answer 1

I think that you may have misread the error message. The only mistake that should cause an error is the fact that you haven't qualified endl with std:: . Are you sure that the error message wasn't about endl ?

Compiling your complete test case, I get the following:

$ g++ test.cpp
test.cpp: In function ‘void hello()’:
test.cpp:11:37: error: ‘endl’ was not declared in this scope
      std::cout << "Hello there!" << endl;
                                     ^
test.cpp:11:37: note: suggested alternative:
In file included from /usr/include/c++/4.8/iostream:39:0,
                 from test.h:1,
                 from test.cpp:1:
/usr/include/c++/4.8/ostream:564:5: note:   ‘std::endl’
     endl(basic_ostream<_CharT, _Traits>& __os)
     ^

Fixing the error by adding the std:: to endl fixes all compile and link errors and gives hello C language linkage as expected.

(Note, there is no harm - and it's probably even clearer - to add extern "C" to the definition of the function hello , but it is not necessary so long as the first visible declaration declares the correct language linkage.)

Answer 2

You should remove extern "C" altogether,
Use the standard namespace.

But if you must compile the function as extern "C" , it is not required you put it before the function definition, only the declaration, which you have done.
But if you'd like to add it to both declaration and definition, it would be fine for you to do so.

Example:

#include "test.h"

using namespace std;    

int main()
{
    hello ();
    return 0;
}

void hello()
{
    cout << "Hello there!" << endl;
}

Answer 3

问题是您在包含文件中声明了extern "C" ，但是它在hello.cpp源文件中，因此它将被编译为c ++，而不是c。