Question

In trying to answer this question I came up with the following code:

#include <string>
#include <iostream> 
#include <algorithm>
#include <vector>

class Sizes
{
public:
    void operator() ( std::vector<int> v ) { 
        sizeVec.push_back( v.size() );  
    }
    std::vector<int> sizeVec;
};

void outFunc (int i) {
    std::cout << " " << i;
}
int _tmain(int argc, _TCHAR* argv[])
{
    std::vector<std::vector<int>> twodVec;

    std::vector<int> vec;
    vec.push_back( 6 );
    twodVec.push_back( vec );
    vec.push_back( 3 );
    twodVec.push_back( vec );
    vec.push_back( 8 );
    twodVec.push_back( vec );
    vec.push_back( 3 );
    twodVec.push_back( vec );

    Sizes sizes;
    std::for_each( twodVec.begin(), twodVec.end(), sizes );
    std::for_each( sizes.sizeVec.begin(), sizes.sizeVec.end(), outFunc );

    return 0;
}

Debugging this shows Sizes::operator() being called and the size of sizeVec increasing with each call as expected. However when the second std::foreach is called the sizeVec is empty… I’ve created a work around involving passing a vector into Sizes but does anyone know what’s going on

Answer 1

std::for_each takes a functor by value, not by reference, so the original object is unaffected. You need to do:

sizes = std::for_each( twodVec.begin(), twodVec.end(), sizes );