How can I avoid "for" loops with an "if" condition inside them with C++?

Boost provides ranges that can be used w/ range-based for. Ranges have the advantage that they don't copy the underlying data structure, they merely provide a 'view' (that is, begin(), end() for the range and operator++(), operator==() for the iterator). This might be of your interest: http://www.boost.org/libs/range/doc/html/range/reference/adaptors/reference/filtered.html

#include <boost/range/adaptor/filtered.hpp>
#include <iostream>
#include <vector>

struct is_even
{
    bool operator()( int x ) const { return x % 2 == 0; }
};

int main(int argc, const char* argv[])
{
    using namespace boost::adaptors;

    std::vector<int> myCollection{1,2,3,4,5,6,7,8,9};

    for( int i: myCollection | filtered( is_even() ) )
    {
        std::cout << i;
    }
}

IMHO it's more straight forward and more readable to use a for loop with an if inside it. However, if this is annoying for you, you could use a for_each_if like the one below:

template<typename Iter, typename Pred, typename Op> 
void for_each_if(Iter first, Iter last, Pred p, Op op) {
  while(first != last) {
    if (p(*first)) op(*first);
    ++first;
  }
}

Usecase:

std::vector<int> v {10, 2, 10, 3};
for_each_if(v.begin(), v.end(), [](int i){ return i > 5; }, [](int &i){ ++i; });

Live Demo