Книга: Standard Template Library Programmer

select1st

select1st<Pair>

Category: functors

Component type: type

Select1st is a function object that takes a single argument, a pair [1], and returns the pair's first element.

Print all of a map's keys.

int main() {
 map<int, double> M;
 M[1] = 0.3;
 M[47] = 0.8;
 M[33] = 0.1;
 transform(M.begin(), M.end(), ostream_iterator<int>(cout, " "), select1st<map<int, double>::value_type>());
 // The output is 1 33 47.
}

Defined in the standard header functional, and in the nonstandard backward-compatibility header function.h. This function object is an SGI extension; it is not part of the C++ standard.

Adaptable Unary Function

There exist some types U and V such that Pair provides the same interface as a pair<U,V>. [1]

unary_function<Pair, Pair::first_type>

All of select1st's members are defined in the Adaptable Unary Function requirements. Select1st does not introduce any new members.

[1] Pair is not actually required to be a pair<U,V> , but merely to support the same interface as pair. In almost all cases the template parameter will be a pair, but it is occasionally useful for it to be something else. One example is a struct that has the members first, second, and third.

identity, select2nd, project1st, project2nd

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