How template deduce const pointer type?

Question

I've tried following codes in cppinsights:

#include <iostream>
#include <string>
#include <type_traits>
#include <vector>

template<typename T>
void printType(T x) {
    std::cout << typeid(x).name() << std::endl;
}

void test() {
  const std::vector<int>::pointer a = new int[2];
  const int* c = new int[2];
  
  printType(a);//line15
  printType(c);//line16
  
  //delete
}

int main() {
  test();
}

The output is like:

/* First instantiated from: insights.cpp:15 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
void printType<int *>(int * x)
{
  std::operator<<(std::cout, typeid(x).name()).operator<<(std::endl);
}
#endif


/* First instantiated from: insights.cpp:16 */
#ifdef INSIGHTS_USE_TEMPLATE
template<>
void printType<const int *>(const int * x)
{
  std::operator<<(std::cout, typeid(x).name()).operator<<(std::endl);
}
#endif

Since std::vector<int>::pointer = int*, why const std::vector<int>::pointer a has been interpreted as int* instead of const int*?

BTW: If the pointer is replaced by value_type, they will be both deduced as int instead of const int.

How compiler deal with const in this type of cases?

Answer 1

Since std::vector<int>::pointer = int*, why const std::vector<int>::pointer a has been interpreted as int* instead of const int*?

Because const std::vector<int>::pointer will be interpreted as int* const instead of const int*. Adding a const-qualifier to a pointer makes itself unmodifiable, not the value it points to.

If you need const int* then you should use std::vector<int>::const_pointer instead of const std::vector<int>::pointer.

Answer 2

why const std::vector<int>::pointer a has been interpreted as int* instead of const int*?

Because std::vector<int>::pointer is int*. A const qualified int* is int* const. It is not const int* aka int const * which is a distinct type. But the constness of the argument has no effect on deduction of the template, so it won't be deduced as int* const.