||Component type: type
Select1st is a function object that takes a single argument,
a pair , and returns the pair's first element.
Print all of a map's keys.
map<int, double> M;
M = 0.3;
M = 0.8;
M = 0.1;
transform(M.begin(), M.end(), ostream_iterator<int>(cout, " "),
// 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++
The function object's argument type.
Adaptable Unary FunctionAdaptable Unary FunctionAdaptable Unary Function
There exist some types U and V such that Pair provides the same
interface as a pair<U,V>. 
Public base classes
All of select1st's members are defined in the
Adaptable Unary Function
requirements. Select1st does not introduce any new members.
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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