std::format Extension

Displaying the address of an arbitrary pointer in C++ 20 fails but succeeds with C++26.

C++20

Only void, const void, and std::nullptr_t pointer types are valid. If you want to display the address of an arbitrary pointer, you must cast it to (const) void*.

// formatPointer20.cpp

#include <format>
#include <iostream>
#include <string>

int main() {

  double d = 123.456789;

  //std::cout << "&d" << std::format("{}", &d) << '\n';  
  std::cout << "static_cast<void *>(&d): " << std::format("{}", static_cast<void *>(&d)) << '\n';     
  std::cout << "static_cast<const void *>(&d): " << std::format("{}", static_cast<const void *>(&d)) << '\n'; 
  std::cout << "nullptr: " << std::format("{}", nullptr) << '\n';           
             
}

In the main function, a double variable named d is initialized with the value 123.456789. The program then prints the address of this variable using std::format.

The first commented-out line attempts to print the address of d directly using std::format("{}", &d). This line is commented out because it would result in a compilation error. The std::format function requires the pointer to be cast to void* or const void* to format it correctly.

The next three lines demonstrate the correct way to format and print pointers using std::format. The first of these lines prints the address of d after casting it to void*. The second line prints the address of d after casting it to const void*. The third line prints the value nullptr, which represents a null pointer.

Each of these lines uses std::cout to print a descriptive message followed by the formatted pointer. The std::format function is used to convert the pointer to a string representation that can be printed to the console.

Here is the output of the program:

Enabling the commented-out line causes a long error message:

C++26

With C++26, you can directly output the pointer:

// formatPointer26.cpp

#include <format>
#include <iostream>
#include <string>

int main() {

  double d = 123.456789;
  double* p = &d;

  std::cout << "&d" << std::format("{:P}", p) << '\n';  
 
}

Honesty, the program does not compile. This contradicts compiler support for C++26 and the proposal P2510R3. Do you know why?

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A solution will be provided in the next article.

What’s Next?

Concurrency will get with C++26 two nice features for lock-free data structures: Read-copy update and hazard pointers.

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