让我们进行以下C 样本：
#include <iostream>
struct X
{
std::string s;
X() : s("X") { }
X(const X& other) : s{other.s} { std::cout << "cpy-ctorn"; }
X(X&& o): s{o.s} { o.s = ""; std::cout << "move-ctorn"; }
X& operator=(const X& other) {
std::cout << "cpy-assignedn";
s = other.s;
return *this;
}
X& operator=(X&& other) {
if (this != &other) {
s = other.s;
other.s = "";
}
std::cout << "move assignedn";
return *this;
}
};
X f(X x) {
std::cout << "f: ";
return x;
}
X g() {
std::cout << "g: ";
X x;
return x;
}
int main() {
X x;
X y;
x = f(X());
y = g();
}
如果我用GCC 4.8.2编译，则有以下结果：
f: move-ctor
move assigned
g: move assigned
我不明白为什么我在调用G函数时未调用复制构建器。
我只是想了解何时调用副本或移动构造函数。
尽管您是正确的，可以从逻辑上确定返回g()的局部变量x的副本/移动，但是C 的有用功能是它可以 elide (即跳过(在许多情况下，即使复制/移动将具有副作用，也是如此。这是其中一种情况。执行时，这被称为命名的返回值优化。
可以说，它比我们移动语义之前的有用性要少，但是仍然很高兴。实际上，C 17在某些(选择(案例中必须进行强制性。
在C 中所有表达式均为：
- lvalue
- prvalue
- xvalue
构建对象
-lvalue
Y x{};
Y y{x}; // copy constructor, x is an lvalue
-prvalue -RVO
使用RVO，默认情况下由GCC启用。这使用复制构造函数，而对象是一次构造的。
X g()
{
X x {};
x.value = 10;
return x;
}
X y {g()}; // X constructor get's called only once to create "y". Also
// y is passed a a reference to g() where y.value = 10.
// No copy/move constructor for optimization "as if" rule
- prvalue -no rvo
没有RVO，在这种情况下，它取决于。如果移动构造函数被明确或隐式删除，则将调用复制构造函数
复制构造函数
struct X { X(const X&) {}}; // implicitly deletes move constructor
// and move assignment, see rule of 5
X g()
{
return X{}; // returns a prvalue
}
X y {g()}; // prvalue gets converted to xvalue,
// "temporary materialization", where the xvalue has an
// identity where members can be copied from. The xvalue
// binds to lvalue reference, the one from copy constructor
// argument
移动构造函数
X { X(X&&) {}}; // explicitly declared move constructor
X g()
{
return X{}; // returns a prvalue
}
X y {g()}; // prvalue gets converted to xvalue,
// "temporary materialization", where the xvalue has an
// identity where members can be moved from. The xvalue
// binds to rvalue reference, the one from move constructor
// argument
-xvalue
X x {};
X y {std::move(x)}; // std::move returns an xvalue, where if move
// constructor is declared will call it, other wise
// copy constructor, similar to explained above for
// prvalue.
复制/移动分配
-lvalue
X x{};
X y{};
x = y; // call copy assignment operator since y is an lvalue.
-prvalue
如果移动分配是明确或隐式删除的，则将调用复制分配运算符。
复制分配
struct X{ X& operator=(const X&); } // implicilty deletes move
// constructor and move assignment,
// see rule of 5
X g()
{
return X{}; // returns a prvalue
}
x = g(); // prvalue gets converted to xvalue,
// "temporary materialization", where the xvalue has an identity
// where members can be copied from. The xvalue binds to lvalue
// reference, the one from copy assignment operator argument
移动分配
struct X{ X& operator=(X&&); } // explicitly declared move assignment operator
X g()
{
return X{}; // returns a prvalue
}
x = g(); // prvalue gets converted to xvalue,
// "temporary materialization", where the xvalue has an identity
// where members can be moved from. The xvalue binds to rvalue
// reference, the one from move assignment operator argument
-xvalue
X x {};
X y {};
x = std::move(x); // std::move returns an xvalue, where if move
// assignment is declared will call it, other
// wise copy assignment, similar to explained
// above for prvalue.
当您使用同一类型的另一个对象实例化对象时，将调用复制构造函数。
例如：
X x;
X y(x);
代码中的最后一行分配了一个从函数返回到已经构造的对象的值。这是通过移动分配完成的。