Question

I have used forward declarations a lot; they help avoid many #includes, improve compilation time and what not. But what if i want to forward-declare a class in the standard library?

// Prototype of my function - i don't want to include <vector> to declare it!
int DoStuff(const std::vector<int>& thingies);

I have heard it’s forbidden/impossible to forward-declare std::vector. Now this answer to an unrelated question suggests to rewrite my code this way:

stuff.h

class VectorOfNumbers; // this class acts like std::vector<int>
int DoStuff(const VectorOfNumbers& thingies);

stuff.cpp

// Implementation, in some other file
#include <vector>
class VectorOfNumbers: public std::vector<int>
{
    // Define the constructors - annoying in C++03, easy in C++11
};

int DoStuff(const VectorOfNumbers& thingies)
{
    ...
}

Now, if i use VectorOfNumbers instead of std::vector<int> in all contexts throughout my project, everything is going to be good, and i don’t need to #include <vector> in my header files anymore!

Does this technique have major disadvantages? Can the gain of being able to forward-declare vector outweigh them?

Answer 1

The reason I wouldn’t do this:

const std::vector<int>& a = a_3rd_party_lib::get_data(); // this type is out of your control
DoStuff(a); // you cannot pass it to this function!