The template default

[marhuum]

What will the template default to if one is declared with it but not supplied when being invoked?

[MikeyBoy]

Are you talking about function templates, or class templates?

[Ganado]

It will default to whatever you told it to default to.

e.g. std::uniform_real_distribution defaults to double.

// Example program
#include <iostream>

template <typename T = int>
class Bing {
  public:
     T bong;
};

int main()
{
    Bing<> bing;
    bing.bong = 3;
    bing.bong += 0.5; // won't do anything; bong is an int
    std::cout << bing.bong << '\n'; // 3
    
    Bing<double> bing2;
    bing2.bong = 3;
    bing2.bong += 0.5;
    std::cout << bing2.bong << '\n'; // 3.5
}

For function templates, it gets more complicated.
I suggest reading: https://en.cppreference.com/w/cpp/language/template_argument_deduction