What other header files include cstdlib?

[jsmithplus]

Other than when you actually include it:

  #include <cstdlib> 

[Ganado]

It's implementation-defined whether or not other header files internally use a particular standard header file.

In general, if the code file needs something that is #include'd with a particular header, then directly include that header file, and don't rely on other unrelated standard header files to include it.

[dutch]

Although the standard dictates particular inclusions of one standard header file in another -- mostly because it's obvious that they would need to be included and often the included header files are ones you don't usually include yourself anyway -- in the end an implementation is allowed to do pretty much whatever it wants. So with the online g++ compiler this compiles:

#include <iostream>
int main() {
    system("ls");             // from <cstdlib> (don't even need std::)
    std::string s{"Hello"};   // from <string>
    std::cout << '\n' << char(tolower(s[0])) << '\n'; // tolower from <cctype>
}

However, you should of course include the proper header files for everything that you need. Unfortunately, the above situation means that you won't always be told if you are missing a header file.

[Duthomhas]

...which it is always a good idea to compile against several compilers.

There is also this Google project:
https://github.com/include-what-you-use/include-what-you-use

The caveat remains, though: if you add a symbol to your program, you should check that the header declaring it is #included at the top of the file.