How can I find out what type the compiler deduced when using the auto
keyword?
Example 1: Simpler
auto tickTime = 0.001;
W
Here's a typeid
version that uses boost::core::demangle
to get the type name at runtime.
#include <string>
#include <iostream>
#include <typeinfo>
#include <vector>
using namespace std::literals;
#include <boost/core/demangle.hpp>
template<typename T>
std::string type_str(){ return boost::core::demangle(typeid(T).name()); }
auto main() -> int{
auto make_vector = [](auto head, auto ... tail) -> std::vector<decltype(head)>{
return {head, tail...};
};
auto i = 1;
auto f = 1.f;
auto d = 1.0;
auto s = "1.0"s;
auto v = make_vector(1, 2, 3, 4, 5);
std::cout
<< "typeof(i) = " << type_str<decltype(i)>() << '\n'
<< "typeof(f) = " << type_str<decltype(f)>() << '\n'
<< "typeof(d) = " << type_str<decltype(d)>() << '\n'
<< "typeof(s) = " << type_str<decltype(s)>() << '\n'
<< "typeof(v) = " << type_str<decltype(v)>() << '\n'
<< std::endl;
}
Which prints this on my system:
typeof(i) = int
typeof(f) = float
typeof(d) = double
typeof(s) = std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >
typeof(v) = std::vector<int, std::allocator<int> >
This SO answer gives a nice function for printing out the name of a type (actually a couple of implementations).
Additionally this free, open-source, header-only library gives a nice way to print out the value and type of chrono::duration
s.
Putting these two utilities together:
#include "chrono_io.h"
#include "type_name.h"
#include <iomanip>
#include <iostream>
int
main()
{
using namespace date;
typedef std::ratio<1, 1> sec;
std::chrono::duration<double, sec > timePerTick2{0.001};
auto nextTickTime = std::chrono::high_resolution_clock::now() + timePerTick2;
std::cout << type_name<decltype(nextTickTime)>() << '\n';
std::cout << std::setprecision(12) << nextTickTime.time_since_epoch() << '\n';
}
This output for me:
std::__1::chrono::time_point<std::__1::chrono::steady_clock, std::__1::chrono::duration<double, std::__1::ratio<1, 1000000000> > >
4.8530542088e+14ns
As Daniel Jour said, read the error message:
... _Tp = std::chrono::time_point<
std::chrono::_V2::system_clock,
std::chrono::duration<
double, std::ratio<1l, 1000000000l> > > ...
A lo-fi trick that doesn't require any prior helper definitions is:
typename decltype(nextTickTime)::_
The compiler will complain that _
isn't a member of whatever type nextTickTime
is.
Here is a way to force a compile error, which shows the type of tickTime
:
struct {} baD = tickTime;