为什么我的复制构造函数被删除了？

Question

给定以下代码：

#include <iostream>
#include <vector>
#include <cstring>
#include <cassert>
#include <sstream>

using std::string;
using std::ostream;
using std::cout;
using std::endl;

typedef std::basic_ostringstream<char> ostringstream;

class Format { //error1
    const char* str;
    ostream& os;
    int index;
    ostringstream savePrint;
public:
    Format(const char* str, ostream& os) :
            str(str), os(os), index(0) {
    }

    ~Format() {
        os << savePrint.str();
    }

    template<class T>
    Format& operator<<(const T& toPrint) {
        while (index < strlen(str) && str[index] != '%') {
            savePrint << str[index++];
        }
        if (index == strlen(str)) {
            throw std::exception();
        }
        assert(str[index] == '%');
        savePrint << toPrint;
        index++;
        return *this;
    }

};

class TempFormat {
    const char* str;
public:
    TempFormat(const char* str) :
            str(str) {
    }
    friend Format operator<<(ostream& os, TempFormat& f) {
        return Format(f.str, os); //error2
    }
};

TempFormat format(const char* str) {
    return TempFormat(str);
}

int main() {
    int year = 2018;
    string hello = "Hello";
    cout << format("% world! The year is %\n") << hello << year; // error3
}

我收到以下错误：

use of deleted function 'std::__cxx11::basic_ostringstream<_CharT, _Traits, _Alloc>::basic_ostringstream(const std::__cxx11::basic_ostringstream<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits; _Alloc = std::allocator]'

并且：

use of deleted function 'Format::Format(const Format&)'

并且：

invalid initialization of non-const reference of type 'TempFormat&' from an rvalue of type 'TempFormat'

谁能解释一下为什么会出现这些错误以及如何解决这些错误？

注意：我想通过此代码实现 printf 的版本（与 % 的连接）。

Answer 1

修复：

#include <sstream> // defines std::basic_ostringstream

// ...

    // basic_ostringstream is movable but no copyable.
    // Make Format moveable.
    Format(Format&&) = default;

// ...

    // Cannot bind a reference to a temporary.
    // R-value reference is required here.
    friend Format operator<<(ostream& os, TempFormat&& f)

为什么我的复制构造函数被删除了？

Why is my copy constructor deleted?

c++

class

c++11

ostream

ostringstream