Capturing breaks my lambda function [duplicate]

Question

I have a function getTotal:

int getTotal( const HitMap& hitMap, bool( *accept)(int chan) )

where the second argument is a bool function specifying which members of the container hitMap should be added to the total.

I'm trying to call it with a lambda. This works:

auto boxresult =
getTotal(piHits, [](int pmt)->bool
{ return (pmt/100) == 1;} );

but this doesn't:

int sector = 100;
auto boxresult =
getTotal(piHits, [sector](int pmt)->bool
{ return (pmt/sector) == 1;} );

I get the error

cannot convert ‘main(int, char**)::<lambda(int)>’ to ‘bool (*)(int)’
for argument ‘2’ to ‘int getTotal(const HitMap&, bool (*)(int))’

from my compiler (GCC 4.6.3). I tried [&sector] and [=sector] but it didn't make any difference.

What am I doing wrong?

Answer 1

When a lambda has a capture clause it can no longer be treated as a function pointer. To correct, use std::function<bool(int)> as the argument type for getTotal():

int getTotal( const HitMap& hitMap, std::function<bool(int)> accept)

Answer 2

The lambda function with capturing is not what you expect, you can use these ways:

template <typename F>
int getTotal( const HitMap& hitMap, F accept )
{

}

or

int getTotal( const HitMap& hitMap, std::function<bool(int)> accept )
{

}

The template based getTotal has better performance. Read more.