C++Guns – RoboBlog

30.09.2015

c++11 move semantics - force it

Filed under: Allgemein — Tags: — Thomas @ 11:09 

// $ g++ --std=c++11 -Wall -Wextra focemove.cpp
// $ ./a.out
// Yes using rvalue, move semantics
// Yes using rvalue, move semantics
// Oh no you forgot std::move somewhere. Deep copy occurs
//
// After delete unwanted ctor
// focemove.cpp: In function ‘void helper(std::vector< int >&&)’:
// focemove.cpp:44:20: error: use of deleted function ‘TestClass::TestClass(std::vector< int >&)’
// focemove.cpp:28:3: error: declared here
//
// testet with 4.7.4 - 5.2.1

#include < vector >
#include < iostream > 
#include < utility >

using namespace std;

// this class store biiiig data in a std::vector.
// If we initialize it with a const reference ctor a deep copy occurs.
// To prevent this, either dont proice a const reference ctor or better:
// forbid it at all.
class TestClass {
public:
  TestClass(vector< int > & other)
    : vec(other)
  {
    cout << "Oh no you forgot std::move somewhere. Deep copy occurs\n";
  }

// do we have to delete a const and non-const version here??
//   TestClass(const vector< int > & other) = delete;
//   TestClass(vector< int > & other) = delete;

  TestClass(vector< int > && other)
    : vec(other)
  {
    cout << "Yes using rvalue, move semantics\n";
  }
private:
  vector< int > vec;
};

void helper(vector< in t> && other) {
  // is other a rvalue or lvalue? It depends on the caller

  // here we forgot to use std::move(other)
  // other is a named rvalue and thus no move semantics are used
  TestClass a(other);

  // comment in this to test after fix TestClass constructor
//   TestClass a(move(other));
}

int main() {
  // this works all fine. All use move semantics
  TestClass(vector< int >());    // using temporaries, unnamed variables
  vector< int > other;
  TestClass(std::move(other)); // named variable other is converted to an unnamed with std::move

  // this call is okay
  helper(std::move(other));

  // this line triggers a error
  // cannot bind ‘std::vector< int >’ lvalue to ‘std::vector< int >&&’
  // helper(other);
  return 0;
}

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