What is std::pair?

Question

What is std::pair for, why would I use it, and what benefits does boost::compressed_pair bring?

Answer 1

compressed_pair uses some template trickery to save space. In C++, an object (small o) can not have the same address as a different object.

So even if you have

struct A { }; 

A's size will not be 0, because then:

A a1; A a2; &a1 == &a2; 

would hold, which is not allowed.

But many compilers will do what is called the "empty base class optimization":

struct A { }; struct B { int x; }; struct C : public A { int x; }; 

Here, it is fine for B and C to have the same size, even if sizeof(A) can't be zero.

So boost::compressed_pair takes advantage of this optimization and will, where possible, inherit from one or the other of the types in the pair if it is empty.

So a std::pair might look like (I've elided a good deal, ctors etc.):

template<typename FirstType, typename SecondType> struct pair {    FirstType first;    SecondType second; }; 

That means if either FirstType or SecondType is A, your pair<A, int> has to be bigger than sizeof(int).

But if you use compressed_pair, its generated code will look akin to:

 struct compressed_pair<A,int> : private A {     int second_;     A first() { return *this; }     int second() { return second_; }  }; 

And compressed_pair<A,int> will only be as big as sizeof(int).

Answer 2

std::pair is a data type for grouping two values together as a single object. std::map uses it for key, value pairs.

While you're learning pair, you might check out tuple. It's like pair but for grouping an arbitrary number of values. tuple is part of TR1 and many compilers already include it with their Standard Library implementations.

Also, checkout Chapter 1, "Tuples," of the book The C++ Standard Library Extensions: A Tutorial and Reference by Pete Becker, ISBN-13: 9780321412997, for a thorough explanation.