是否可以知道是否包含标题

Question

In the source code is it possible to know if a header is included?

This an example of what I need :

#include<iostream>
using namespace std;

int main()
{
    char headname[4];
    cout<<"Enter a header name : ";
    cin>>headname;

    #ifdef headname
        cout<<headname<<" Defined"<<endl;
    #else
        cout<<headname<<" Not defined"<<endl;
    #endif

    return 0;
}

For example, if I enter "iostream", the output should be "iostream Defined".

Answer 1

Yes. Headers usually use include guards such as:

#ifndef MY_HEADER_INCLUDED
#define MY_HEADER_INCLUDED

// [...]

#endif

On my Gentoo Linux / GCC system, looking at the iostream header I see:

#ifndef _GLIBCXX_IOSTREAM
#define _GLIBCXX_IOSTREAM 1

so you could check for _GLIBCXX_IOSTREAM . If you're not using GCC, open your iostream header file and see what macros they might have defined.

It should also be pointed out that cout belongs to the iostream header, so when _GLIBCXX_IOSTREAM (in my case) is not defined, the code will also fail to compile. But you can use printf() for the test.

Answer 2

Sure it's possible:

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>

int main(void) {
    std::vector<std::string> includes;
    includes.push_back("iostream");
    includes.push_back("vector");
    includes.push_back("string");
    includes.push_back("algorithm");

    std::string user_input;
    std::cout << "Enter a header name: ";
    std::cin >> user_input;

    if ( std::find(includes.begin(), includes.end(), user_input) !=
            includes.end() ) {
        std::cout << user_input << " is included." << std::endl;
    } else {
        std::cout << user_input << " is not included." << std::endl;
    }

    return 0;
}

Output:

paul@local:~/src/cpp/scratch$ ./incl
Enter a header name: iostream
iostream is included.
paul@local:~/src/cpp/scratch$ ./incl
Enter a header name: map
map is not included.
paul@local:~/src/cpp/scratch$

Answer 3

First off, checking for a header, "at runtime" and "at compile time" are no different since #include is executed at compile time along with any #ifdef . A #include essentially does a copy-paste of the header at the top of your .cpp file. Razvan Cojocaru pointed out that you can use the #ifdef to check weather _GLIBCXX_IOSTREAM is defined. Here is a little example of where this could be used:

class Flagger
{
    typedef unsigned long ulong;
    public:
        Flagger (ulong flags = 0)    : f(flags) { ; }
        Flagger (const Flagger& cpy) : f(cpy.f) { ; }

        void clear  (ulong flags) { f &= ~flags; }
        void set    (ulong flags) { f |= flags;  }
        void toggle (ulong flags) { f ^= flags;  }

        bool get    (ulong flags) { return f & flags; }

#ifdef _GLIBCXX_OSTREAM
        friend std::ostream& operator << (std::ostream &out, const Flagger& f)
                { /* print it how you want it*/ }
#endif

    private:
        ulong f;
};

However, this is probably a bad idea for several reasons:

1. The user would need to include the above header after including iostream or the compiler would delete the function.
2. If at any point in time the makers of the iostream library decides to change the #define name, the function will be deleted. Likewise, if someone is using a different version of iostream with a different #define tag, the function is deleted.
3. In the above example, it is not too different to just include the library yourself. If some random user does not use the library, the final size of their program will not be much different and functionality will not have changed at all.

So basically, yes it is possible , but not practical . Especially for long-term maintenance. The benefits don't outweigh the danger. Just include the library in question yourself.

As stated by others here, it would be helpful if we knew what your final outcome is. The place you are planning to use this in most likely has a better solution.

Answer 4

Yes, A quick hack to do this is:

All you need to do is go to the source code header file. And access any member (any constant defined anywhere in source code of header file) via -

cout<<ACCESSED_MEMBER

now compile and run the program, if you got it printed on console then congrats, the file is included successfully in your code.

Note:1 To find particular source file for header. you can type on terminal as

locate <header_file_name> You will get the location of file. now check for any member by opening it in vi or nano or gedit or any editor of your choice.

Note:2 Do not make any changes to this header file if it is not owned by you.