Does pair.first return a reference to the first value?

Question

In the C++ standard library, there is an object called the pair. Pair.first and Pair.second return the first and second values of the pair object, respectively. I want to increment the first value by one under a conditional (my pair is a pair). It's throwing "needs 1-value" when I try to do this. This could be because somewhere along the way I am not passing in a value by reference, but it could also be that .first does not return the first value of the pair by reference. Does anyone know if it does?

Answer 1

first is not a function, it is an object itself, a data member of struct pair. The name of an object is an lvalue, equivalent to a reference.

"Lvalue" is a language term denoting something that can go on the left-hand side of an assignment, i.e. a value can be assigned to it, or it has a meaningful address in memory.

The result of a conditional expression is an lvalue if both its alternatives are lvalues of the same type.

For example,

pair< int, int > p;
flag? p.first : p.second = 34; // ok

flag? p.first : 3 = 15; // error: 3 is not an lvalue; 3 = 15 is nonsense

pair< int, short > q;
flag? q.first : q.second = 12; // error: different types

Note, the opposite of "lvalue" is "rvalue," for right-hand side of an assignment. These terms are borrowed from the C language.

Remember: the lvalue of an expression is where it is stored, the rvalue is its content. Which of these an expression meaningfully has decide its value category.