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C++17 in Detail: A Deep Dive

Debug Printing

Now, we shall study a print function which uses several new facilities of C++.

We'll cover the following...

In this lesson we’ll examine a few more functions available in C++17. Do not worry about the code itself.

Here are all the functions and utilities you need to focus on:

  • if constexpr
  • _v templates
  • linePrinter
C++ 17
#include <iostream>
template<typename T> void linePrinter(const T& x) {
  if constexpr (std::is_integral_v<T>)
    std::cout << "num: " << x << '\n';
  else if constexpr (std::is_floating_point_v<T>) {
    const auto frac = x - static_cast<long>(x);
    std::cout << "flt: " << x << ", frac " << frac << '\n';
  }
  else if constexpr(std::is_pointer_v<T>) {
    std::cout << "ptr, ";
    linePrinter(*x);
  }
  else
    std::cout << x << '\n';
}
template<typename ... Args> void printWithInfo(Args ... args) {
  (linePrinter(args), ...); // fold expression over the comma operator
}
int main () {
  int i = 10;
  float f = 2.56f;
  printWithInfo(&i, &f, 30);
}

Here you can see the following features:

• Line 5, 7, 11: if constexpr - to discard code at compile time, used to match the template parameter.

• Line 5, 7, 11: _v variable templates for type traits - no need to write std::trait_name<T>::value.

• Line 20: Fold Expressions inside printWithInfo. This feature simplifies variadic templates. In the example we invoke linePrinter() over all input arguments.

What we have seen here is just a small shadow of what’s to come ahead. See you in the next section!