Can std::string be used without #include ?

问题:

This question already has an answer here:

• Why does omission of “#include <string>” only sometimes cause compilation failures? 5 answers

Here is my code:

#include <iostream>

int main(int argc, char const *argv[])
{
    std::string s = "hello";
    std::cout << s.size() << std::endl;
    return 0;
}

To my surprise, I can compile and run it with clang++, though I even don't add #include <string>.

So, is it necessary to add #include <string> in order to use std::string ?

回答1:

Your implementation's iostream header includes string. This is not something which you can or should rely on. If you want to use std::string, you should always #include <string>, otherwise your program might not run on different implementations, or even in later versions of your current one.

回答2:

Some header might include other headers, but that's an implementation issue and not something you can count on. Always explicitly include the headers you need.

回答3:

In short: yes, it is necessary.

Parts of standard library often use other parts, so <string> was included somehow through <iostream>, and your code compiles nicely.

If you accidentally decide that you don't need <iostream> anymore and remove that include, <string> will be implicitly removed, too, and you get a confusing compilation error. That's why it is a good practice to put all necessary includes.

回答4:

iostream includes <string>. It ripples through.

Well actually that's implementation dependant and not a guarantee. You should explicitly include the headers you need.