Why Doesn't My C 11 Move Constructor Get Called in This Case?

C 11 Move Constructor Not Called, Default Constructor Preferred

Problem

Consider the following class:

<code class="cpp">class X {
public:
    explicit X(char* c) { cout << "ctor" << endl; init(c); };
    X(X& lv)  { cout << "copy" << endl;  init(lv.c_); };
    X(X&& rv) { cout << "move" << endl;  c_ = rv.c_; rv.c_ = nullptr; };

    const char* c() { return c_; };

private:
    void init(char *c) { c_ = new char[strlen(c)+1]; strcpy(c_, c); };
    char* c_;
};</code>

And its usage:

<code class="cpp">X x("test");
cout << x.c() << endl;
X y(x);
cout << y.c() << endl;
X z( X("test") );
cout << z.c() << endl;</code>

The output is:

ctor
test
copy
test
ctor   <-- why not move?
test

In VS2010, using default settings, one would expect the last object (z) to be move-constructed instead of default-constructed. Using X z( move(X("test")) ) results in the expected output: ctor move test. Could this be a case of NRVO?

Question

Should the move constructor be called according to the C 11 standard? If so, why isn't it called?

Answer

The observed behavior is due to copy elision. The compiler can directly construct a temporary into a target it is to be copied/moved into, thus omitting the copy/move constructor and destructor calls.

The situations where copy elision can be applied are outlined in §12.8.32 of the C 11 standard:

• In a function with a class return type, if the return expression is a non-volatile automatic object with the same type as the return type.
• In a throw-expression, if the operand is a non-volatile automatic object with a scope not extending beyond the enclosing try-block.
• When a class object that has not been bound to a reference would be copied/moved to a class object with the same type.
• When the exception handler declares an object of the same type as the exception object by treating the exception-declaration as an alias.

In this case, the compiler is able to elide the copy or move operation between the temporary X("test") and the target z, resulting in the observed behavior.

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