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C++17 in Detail: A Deep Dive

Path Composition

Explore how to compose file paths in C++17 using the filesystem library's append and concat functions. Understand platform-dependent behaviors, path format conversions, and stream operators to work effectively with both POSIX and Windows systems.

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We have two methods that allow to compose a path:

  • path::append() - adds a path with a directory separator.
  • path::concat() - only adds the ‘string’ without any separator.

The functionality is also available with operators /, /= (append), + and += (concat).

For example:

C++ 17
#include <iostream>
#include <filesystem>
using namespace std;
int main(){
  
  namespace fs = std::filesystem;
  fs::path p1("C:\\temp"); p1 /= "user";
  p1 /= "data";
  cout << p1 << '\n';
  fs::path p2("C:\\temp\\"); p2 += "user";
  p2 += "data";
  cout << p2 << '\n';
}

However, appending a path has several rules that you have to be aware of.

For example, if the other path is absolute or the other path has a root-name, and the root-name is different from the current path root name. Then the append operation will replace this.

auto resW = fs::path{"foo"} / "D:\"; // Windows
auto resP = fs::path{"foo"} / "/bar"; // POSIX
// resW is "D:\" now
// resP is now "/bar"

In the above case resW and resP will contain the value from the second operand. As ...