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What is the difference between using a struct with two fields and a pair?

Dear all,

I have a little question about pairs and struct. Is there any advantage to use a std::pair instead of a struct with two cells ?
I have used pairs for a while but the main problem is readability :
If you want to represent for example a duple (int “label”, double “value”) you can use either a :

typedef std::pair<int,double> myElem;

or a

typedef struct {
    int label;
    double value;
} myElem;

The code becomes more readable if your statements have a “semantic” sense (you will always know what x.label is. that’s not the case with x.first).

However, I guess there is an advantage using pair. Is it more performant or something else?

Answer 1

A pair is implemented as a templated struct. It provides you with a shorthand for creating a (typically heterogenous) pair. Also, there are some constraints on the types that you can use with a pair:

Type requirements

T1 and T2 must both
be models of Assignable. Additional
operations have additional
requirements. Pair’s default
constructor may only be used if both
T1 and T2 are DefaultConstructible,
operator== may only be used if both T1
and T2 are EqualityComparable, and
operator< may only be used if both T1
and T2 are LessThanComparable.

(from SGI STL std::pair documentation)

Defining your own POD may make sense if the types do not follow any of these constraints or if you do not care about them.

Finally, I guess it is a matter of personal choice/coding style.