我正在做C++ Primer 5th Edition的练习，就像：
练习 13.50：将 print 语句放入 String类的移动操作中，然后重新运行第 13.6.1 节（第 534 页）中练习 13.48 中的程序，该程序使用向量查看何时避免复制。（第 544 页）
这String是一个练习类，其行为类似于std::string不使用任何模板。String.h文件：
class String
{
public:
//! default constructor
String();
//! constructor taking C-style string i.e. a char array terminated with'\0'.
explicit String(const char * const c);
//! copy constructor
explicit String(const String& s);
//! move constructor
String(String&& s) noexcept;
//! operator =
String& operator = (const String& rhs);
//! move operator =
String& operator = (String&& rhs) noexcept;
//! destructor
~String();
//! members
char* begin() const { return elements; }
char* end() const { return first_free; }
std::size_t size() const {return first_free - elements; }
std::size_t capacity() const {return cap - elements; }
private:
//! data members
char* elements;
char* first_free;
char* cap;
std::allocator<char> alloc;
//! utillities for big 3
void free();
};
默认、复制和移动构造函数的实现String.cpp：
//! default constructor
String::String():
elements (nullptr),
first_free (nullptr),
cap (nullptr)
{}
//! copy constructor
String::String(const String &s)
{
char* newData = alloc.allocate(s.size());
std::uninitialized_copy(s.begin(), s.end(), newData);
elements = newData;
cap = first_free = newData + s.size();
std::cout << "Copy constructing......\n";
}
//! move constructor
String::String(String &&s) noexcept :
elements(s.elements), first_free(s.first_free), cap(s.cap)
{
s.elements = s.first_free = s.cap = nullptr;
std::cout << "Move constructing......\n";
}
Main.cpp：
int main()
{
std::vector<String> v;
String s;
for (unsigned i = 0; i != 4; ++i)
v.push_back(s);
return 0;
}
输出：
Copy constructing......
Copy constructing......
Copy constructing......
Copy constructing......
Copy constructing......
Copy constructing......
Copy constructing......
可以看出，根本没有调用移动构造函数。为什么向量分配更多内存时不调用移动构造函数？
更新：
编译器信息：
gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1)
main.cpp具有打印能力及其输出：
int main()
{
std::vector<String> v;
String s;
for (unsigned i = 0; i != 4; ++i)
{
std::cout << v.capacity() << "\n";
v.push_back(s);
}
return 0;
}
输出：
0
Copy constructing......
1
Copy constructing......
Copy constructing......
2
Copy constructing......
Copy constructing......
Copy constructing......
4
Copy constructing......