需要一些有关 C++ 中“交换”功能的帮助

Question

I am new to C++ programming. I am trying to compile another person's program, and the software reported a bug that I don't know how to solve (the software I use is Visual Studio 2019).

The line of code with bug is:

swap(allBeamPoints, unordered_map<int, vector<LidarPoint>>());

The definition for variable allBeamPoints is:

unordered_map<int, vector<LidarPoint>>allBeamPoints

The error information is:

Error   C2665   'std::swap': none of the 2 overloads could convert all the argument types   

and

Error (active)  E0304   no instance of overloaded function "swap" matches the argument list 

However, if I type the following codes, no bug will be reported:

unordered_map<int, vector<LidarPoint>> allBeamPoints_new;

swap(allBeamPoints, allBeamPoints_new);

Does anyone know what the problem is? Did I fail to link some of the required libraries? How should I check those libraries?

Answer 1

Try this way. ( Here )

// unordered_map::swap
#include <iostream>
#include <string>
#include <unordered_map>

int main ()
{
  std::unordered_map<std::string,std::string>
     first = {{"Star Wars","G. Lucas"},{"Alien","R. Scott"},{"Terminator","J. Cameron"}},
     second  = {{"Inception","C. Nolan"},{"Donnie Darko","R. Kelly"}};

  first.swap(second);

  std::cout << "first: ";
  for (auto& x: first) std::cout << x.first << " (" << x.second << "), ";
  std::cout << std::endl;

  std::cout << "second: ";
  for (auto& x: second) std::cout << x.first << " (" << x.second << "), ";
  std::cout << std::endl;

  return 0;
}

Answer 2

The older MSVC compilers allowed the binding of an anonymous temporary to a non- const reference function parameter. That's not standard C++.

So

swap(allBeamPoints, unordered_map<int, vector<LidarPoint>>());

which had the effect of clearing allBeamPoints was reasonably idiomatic in code that only targeted Microsoft platforms.

It's wrong and not portable as you now observe. Use

allBeamPoints.clear();

instead to clear the map. Your second snippet compiles since allBeamPoints_new is not an anonymous temporary.