std::transform is the same as map. The idea is to apply a function to each element in between the two iterators and obtain a different container composed of elements resulting from the application of such a function. You may want to use it for, e.g., projecting an object’s data member into a new container. In the following, std::transform is used to transform a container of std::strings in a container of std::size_ts.
std::vector<std::string> names = {"hi", "test", "foo"};
std::vector<std::size_t> name_sizes;
std::transform(names.begin(), names.end(), std::back_inserter(name_sizes), [](const std::string& name) { return name.size();});
On the other hand, you execute std::for_each for the sole side effects. In other words, std::for_each closely resembles a plain range-based for loop.
Back to the string example:
std::for_each(name_sizes.begin(), name_sizes.end(), [](std::size_t name_size) {
std::cout << name_size << std::endl;
});
Indeed, starting from C++11 the same can be achieved with a terser notation using range-based for loops:
for (std::size_t name_size: name_sizes) {
std::cout << name_size << std::endl;
}