Using Move Semantics In Own Code¶

Using `String`: `class Person`¶

Idea: constructor taking person’s firstname and lastname
Passing temporaries right into members ⟶ implement move constructor

code/person-10.h¶

#ifndef PERSON_H
#define PERSON_H

#include "string-30.h"

struct Person
{
    Person(String&& firstname, String&& lastname)
    : firstname(firstname), 
      lastname(lastname) {}

    String firstname;
    String lastname;
};

#endif

code/person-suite-bogus.cpp¶

#include <gtest/gtest.h>
#include <cstring>

#include "person-10.h"

TEST(person_suite, bogus)
{
    Person p("Joerg", "Faschingbauer");  // <--- temporary String object -> move ctor

    ASSERT_EQ(strcmp(p.firstname.c_str(), "Joerg"), 0);
    ASSERT_EQ(strcmp(p.lastname.c_str(), "Faschingbauer"), 0);
}

code/person-10.h:9:7: error: use of deleted function ‘String::String(const String&)’
    9 |     : firstname(firstname), lastname(lastname) {}
      |       ^~~~~~~~~~~~~~~~~~~~

What?¶

Remember: inside a function, rvalue reference parameters are lvalues
They have names
- ⟶ can determine its address
- … which is the definition of an lvalue
Solution: convert into rvalue reference
⟶ std::move

Using Rvalue References Inside Function: `std::move`¶

code/person-20.h¶

#ifndef PERSON_H
#define PERSON_H

#include "string-30.h"

struct Person
{
    Person(String&& firstname, String&& lastname)   // <--- parameters have names -> lvalues
    : firstname(std::move /*!!*/ (firstname)),      // <--- request String's move contructor
      lastname(std::move /*!!*/ (lastname)) {}

    String firstname;
    String lastname;
};

#endif