Modern way to filter STL container?

See the example from cplusplus.com for std::copy_if:

std::vector<int> foo = {25,15,5,-5,-15};
std::vector<int> bar;

// copy only positive numbers:
std::copy_if (foo.begin(), foo.end(), std::back_inserter(bar), [](int i){return i>=0;} );

std::copy_if evaluates the lambda expression for every element in foo here and if it returns true it copies the value to bar.

The std::back_inserter allows us to actually insert new elements at the end of bar (using push_back()) with an iterator without having to resize it to the required size first.

In C++20, use filter view from the ranges library: (requires #include <ranges>)

// namespace views = std::ranges::views;
vec | views::filter([](int a){ return a % 2 == 0; })

lazily returns the even elements in vec.

(See [range.adaptor.object]/4 and [range.filter])

This is already supported by GCC 10 (live demo). For Clang and older versions of GCC, the original range-v3 library can be used too, with #include <range/v3/view/filter.hpp> (or #include <range/v3/all.hpp>) and the ranges::views namespace instead of std::ranges::views (live demo).

A more efficient approach, if you don't actually need a new copy of the list, is remove_if, which actually removes the elements from the original container.

I think Boost.Range deserves a mention too. The resulting code is pretty close to the original:

#include <boost/range/adaptors.hpp>

// ...

using boost::adaptors::filtered;
auto filteredElements = elements | filtered([](decltype(elements)::value_type const& elm)
    { return elm.filterProperty == true; });

The only downside is having to explicitly declare the lambda's parameter type. I used decltype(elements)::value_type because it avoids having to spell out the exact type, and also adds a grain of genericity. Alternatively, with C++14's polymorphic lambdas, the type could be simply specified as auto:

auto filteredElements = elements | filtered([](auto const& elm)
    { return elm.filterProperty == true; });

filteredElements would be a range, suitable for traversal, but it's basically a view of the original container. If what you need is another container filled with copies of the elements satisfying the criteria (so that it's independent from the lifetime of the original container), it could look like:

using std::back_inserter; using boost::copy; using boost::adaptors::filtered;
decltype(elements) filteredElements;
copy(elements | filtered([](decltype(elements)::value_type const& elm)
    { return elm.filterProperty == true; }), back_inserter(filteredElements));

Improved pjm code following underscore-d suggestions:

template <typename Cont, typename Pred>
Cont filter(const Cont &container, Pred predicate) {
    Cont result;
    std::copy_if(container.begin(), container.end(), std::back_inserter(result), predicate);
    return result;
}

Usage:

std::vector<int> myVec = {1,4,7,8,9,0};

auto filteredVec = filter(myVec, [](int a) { return a > 5; });