C++17 Features: std::apply and fold expressions
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std::apply and fold expressions in C++17: simplifying parameter packs
Often, I find myself in a scenario where I need to “iterate” over the elements inside a tuple. Prior to C++17, the basic idea behind accomplishing this was through the use of parameter packs and std::index_sequence. However, with the introduction of std::apply and fold expressions, this task is much simpler.
#include <functional>
#include <iostream>
#include <typeinfo>
#include <initializer_list>
#include <tuple>
#include <vector>
#include <set>
#include <unordered_set>
#include <string_view>
// stolen from https://stackoverflow.com/questions/81870/is-it-possible-to-print-a-variables-type-in-standard-c/56766138#56766138
// get useful type information without needing to use c++filt
// does the unmangling for you!
template <typename T>
constexpr auto type_name() {
std::string_view name, prefix, suffix;
#ifdef __clang__
name = __PRETTY_FUNCTION__;
prefix = "auto type_name() [T = ";
suffix = "]";
#elif defined(__GNUC__)
name = __PRETTY_FUNCTION__;
prefix = "constexpr auto type_name() [with T = ";
suffix = "]";
#elif defined(_MSC_VER)
name = __FUNCSIG__;
prefix = "auto __cdecl type_name<";
suffix = ">(void)";
#endif
name.remove_prefix(prefix.size());
name.remove_suffix(suffix.size());
return name;
};
int main(){
std::initializer_list initializer = {1, 2, 3, 4, 3, 2, 1};
auto createSet = [=](){
return std::set<int>(initializer);
};
auto createUnorderedSet = [=](){
return std::unordered_set<int>(initializer);
};
auto createVector = [=](){
return std::vector<int>(initializer);
};
auto createMultiSet = [=](){
return std::multiset<int>(initializer);
};
auto printContainer = [](auto& containerCreator){
auto container = containerCreator();
std::cout << "Iterating over container of type : " << type_name<decltype(container)>() << "\n";
for(auto&& entry : container){
std::cout << "\t" << entry << "\n";
}
};
std::apply([=](auto&&... containerCreator){
(printContainer(containerCreator),...);
}, std::tuple{createSet, createUnorderedSet, createVector, createMultiSet});
}
That’s it – just:
std::apply([=](auto&&... containerCreator){
(printContainer(containerCreator),...);
}, std::tuple{createSet, createUnorderedSet, createVector, createMultiSet});
Check out this example on godbolt: https://godbolt.org/z/dKEr6v43v.