C++ Brain Teasers: Exercise Your Mind/

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Discussion: A Constant Struggle

Discussion: A Constant Struggle

Execute the code to understand the output and gain insights into template argument deduction.

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Run the code

Now, it’s time to execute the code and observe the output.

C++
#include <iostream>
#include <type_traits>
template <typename T>
void byValue(T t)
{
    std::cout << std::is_const_v<T>; // true if T is const
}
template <typename T>
void byReference(T &t)
{
    std::cout << std::is_const_v<T>; // true if T is const
}
int main()
{
    int nonConstInt = 0;
    const int constInt = 0;
    byValue(nonConstInt);
    byValue(constInt);
    byReference(nonConstInt);
    byReference(constInt);
}

Understanding the output

The two function templates byValue and byReference take their parameters by value and by reference, respectively. We then call these two function templates with a non-const and a const int as arguments. In which cases is T deduced as const?

Template parameter

Before we start digging into the deduction rules, let’s clarify two concepts that can be easily mixed up. The T in template <typename T> is called the template parameter. The goal of template argument deduction is to find T. On the other hand, the T and T & in the function signatures byValue(T ...