我有以下代码:
template<class T = char>
class String
{
public:
// Default constructor
String()
: buffer(nullptr),
len(0)
{
cout << "Default constructor" << endl;
}
// Constructor
String(const char* s)
{
cout << "Constructor (const char*)" << endl;
//...
}
// Virtual destructor.
virtual ~String()
{
cout << "Destructor" << endl;
len = 0;
delete[] buffer;
}
// Copy constructor
String(const String& s)
{
cout << "Copy constructor" << endl;
buffer = new T[s.len];
std::copy(s.buffer, s.buffer + s.len, buffer);
len = s.len;
}
// Copy assignment operator (uses copy and swap idiom)
String& operator=(String s)
{
cout << "Copy assignment operator (copy and swap idiom)" << endl;
std::swap(buffer, s.buffer);
return *this;
}
// Move constructor
String(String&& s)
{
cout << "Move constructor" << endl;
}
// compound assignment (does not need to be a member,
// but often is, to modify the private members)
String& operator+=(const String& rhs)
{
cout << "operator+=" << endl;
//...
return *this; // return the result by reference
}
// friends defined inside class body are inline and are hidden from non-ADL lookup
// passing lhs by value helps optimize chained a + b + c
// otherwise, both parameters may be const references
friend String operator+(String lhs, const String& rhs)
{
cout << "operator+" << endl;
lhs += rhs; // reuse compound assignment
return lhs; // return the result by value (uses move constructor)
}
private:
T* buffer;
size_t len;
};
int main()
{
String<> s("Hello ");
String<> s2("World");
// call copy constructor first?
String<> s3 = s + s2;
return 0;
}
输出是:
Constructor (const char*)
Constructor (const char*)
Copy constructor
operator+
operator+=
Move constructor
Destructor
我的问题是为什么立即调用复制构造函数:
String<> s3 = s + s2;
答案 0 :(得分:7)
s
的值副本由friend String operator+(String lhs, const String& rhs)
占用,主要是因为s
不是匿名临时的,因此不适合移动构建。获取该值复制需要复制构造函数。
答案 1 :(得分:0)
根据我的理解,调用复制构造函数来接收+
运算符的返回值。