ptr as class member and move semantics fail to compile with clang
I can't get clang (Apple LLVM version 4.2 (clang-425.0.28)) to compile these classes:
struct A {
int f(){return 2;}
};
class Cl{
std::unique_ptr<A> ptr;
public:
Cl(){ptr = std::unique_ptr<A>(new A);}
Cl(const Cl& x) : ptr(new A(*x.ptr)) { }
Cl(Cl&& x) : ptr(std::move(x.ptr)) { }
Cl(std::unique_ptr<A> p) : ptr(std::move(p)) { }
void m_ptr(std::unique_ptr<A> p){
ptr = std::unique_ptr<A>(std::move(p));
}
double run(){return ptr->f();}
};
I would like to run the constructor as follows:
std::unique_ptr<A> ptrB (new A);
Cl C = Cl(ptrB);
but if I do this I get the following compiler error: ../src/C++11-2.cpp:66:10: error: call to implicitly-deleted copy constructor of 'std::unique_ptr' C.m_ptr(ptrB);
I can solve the compiler problem by running Cl(std::move(ptrB)) but this doesn't actually move the ownership of A away from ptrB: I can still run ptrB->f() without causing a run-time crash... Secondly, the constructor is not very satisfying, since I want to hide the implementation of std::move in the class interface.
Thanks in advance.
Since ptrB is passed by value to Cl's copy constructor, a call to Cl(ptrB) tries to create a copy of ptrB which in turn calls a (obviously disabled) copy constructor of unique_ptr. In order to avoid creating an extra copy of ptrB, do the following:
Cl C = Cl(std::unique_ptr<A>(new A)); //A temporary is created on initialization, no extra copy steps performed
Or:
std::unique_ptr<A> ptrB (new A);
Cl C = Cl(std::move(ptrB)); //Move semantics used. Again, no extra copy steps
Or, use pass by reference (rvalue or lvalue) in your copy constructor:
class Cl{
//...
public:
//...
Cl(std::unique_ptr<A> &p) : ptr(std::move(p)) { }
//...
};
std::unique_ptr<A> ptrB (new A);
Cl C = Cl(ptrB);
PS Oh and by the way: the objects stay in unspecified, but valid state after std::move(). I believe that means you can still call ptrB->f(), and it is guaranteed to return 2 :)
链接地址: http://www.djcxy.com/p/59676.html上一篇: 在C ++ 03中模拟显式强制转换