涉及 iostream 和 wstring 的奇怪行为 C++

Question

I have two files, my_program.cpp and its header my_program.h .

my_program.cpp contains only this:

#include "my_program.h"
using namespace std;

my_program.h contains a pointer to a function which returns a wstring , as follows:

using namespace std;
typedef wstring (*my_function)(wstring, int, int, int, int);

The program doesn't compile in this state (stops at typedef... saying that ISO C++ forbids declaration of 'wstring' with no type) but if I add #include <iostream> before #include "my_program.h" in the.cpp file, the program miraculously compiles.

Why does this happen? I just hope I didn't make a foolish mistake and I'm now going to be laughed at.

Answer 1

You really should include <string> .

From the sound of things, your compiler's <iostream> happens to include <string> , so it works, but on a different compiler it may not. C++ allows a standard header to include other standard headers, but doesn't require it. In some cases, you get only a declaration of the class, so some things work, and others don't.

At least in my experience, this is also an issue that's likely to change from one version of a compiler to the next, so even if you don't intend to port to anything else, your code may quit working just due to a seemingly trivial upgrade unless you include the right header.

Answer 2

Before you add #include <iostream> , the compiler has no idea what wstring is -- hence the error

After you include the iostream header, (which defines std::wstring -- possibly via other headers it includes), the compiler knows what wstring refers to and can properly parse the typedef .

One thing to keep in mind is that the compiler only cares about what's called the compilation unit (.cpp files), not the headers; the headers come into play when a compilation unit includes them (think of verbatim copy-paste). Therefore you can include iostream (or as others pointed out <string> ) anywhere before the typedef , to make it work -- from this perspective, including it before the header counts the same as including it at the first line of the header.

It is generally a good idea, however, to make headers as self consistent as possible (so in your case include the relevant headers in your header, instead of the.cpp), so the includers do not need to remember what other headers to include as well to make it work as intended.