C ++代码缺少标头运行，为什么？

Question

I just realized that I am supposed to include the #include<cstdlib> required by abs() for the abs() function.

 #include<iostream>
    using namespace std;
    int main()
    {
        int result;
        result = abs(-10);
        cout << result << "\n";
        return 0;

    }

Why does this code still work, even though I forgot the important header ( #include<cstdlib> )?

Answer 1

That's because iostream indirectly includes definition for abs() . It is allowed by the Standard, but should not be relied upon, because it's implementation-dependant (ie your code may not compile on some other compilers).