Re: Deduce non-type template parameter return value

From:

88888 Dihedral <dihedral88888@googlemail.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 8 Feb 2012 00:18:37 -0800 (PST)

Message-ID:

<29415009.344.1328689117791.JavaMail.geo-discussion-forums@prfs3>

=E5=9C=A8 2012=E5=B9=B42=E6=9C=888=E6=97=A5=E6=98=9F=E6=9C=9F=E4=B8=89UTC+8=
=E4=B8=8A=E5=8D=888=E6=97=B659=E5=88=8641=E7=A7'=EF=BC=8CChris McAce=E5=
=86=99=E9=81=93=EF=BC=9A

Hi,

I've written a function template for applying a free function to every
element of a std::vector:

#include <vector>

template<class T, T F(T)>
std::vector<T> map(const std::vector<T>& from)
{
    std::vector<T> to;
    to.reserve(from.size());
    for (typename std::vector<T>::const_iterator iter = from.begin();
iter != from.end(); ++iter) {
        to.push_back(F(*iter));
    }
    return to;
}

This can be used as follows:

#include <cmath>

int main()
{
    std::vector<float> angles;
    angles.push_back(1);
    angles.push_back(2);
    angles.push_back(3);
    std::vector<float> sines = map<float, std::sin>(angles);
}

As you can see, std::sin is passed as a non-type template parameter.
However, I'd like to get rid of the the first parameter (float in the
example) so the call would become map<std::sin>(angles). Is there a
way to do this in C++03 or C++11?

Please note that I don't want to pass function pointers at run-time;
the mapping function (here, std::sin) is known at compile-time so I
want to take advantage of that.

Thanks,
Chris

A template function that accepts any data type and any known function to
operate on a vector of objects to be compiled is different from
an unknown function pointer passed in by the caller part in the run time.

Any object in a vector can be mapped as in the above code.
The constructor has to be called for each new object pushed into the result=

vector to.

Note if the object is very large and the length of the vector is large, too
then the above code might run into problems in the heap space.