c++模板复制构造函数不可用或显式

Question

我写了一个带有模板复制构造函数的代码，以便更好地理解这个概念，因为我是新手，但以下代码无法编译

#include <iostream>
#include <vector>

using namespace std;

template <typename T>
class Grid
{
public:
    explicit Grid(size_t inWidth = kDefaultWidth, size_t inHeight = kDefaultHeight);
    virtual ~Grid();

    template <typename E>
    Grid(const Grid<T>& src);

    static const size_t kDefaultWidth = 10;
    static const size_t kDefaultHeight = 10;
    std::vector<std::vector<T>> mCells;
    size_t mWidth, mHeight;
};

template <typename T>
template <typename E>
Grid<T>::Grid(const Grid<T>& src)
{
    cout << "Copy constructor working " << endl;

}

int main()
{
    Grid<double> myDoubleGrid;
    Grid<double> newDoubleGrid(myDoubleGrid);
    return 0;
}

在 visual studio 中编译上述代码时出现以下错误:-

错误:-

Severity Code Description Project File Line Suppression State Error C2558 class 'Grid': no copy constructor available or copy constructor is declared 'explicit'

我尝试用 E 替换参数的模板，但它显示了更多错误(奇怪的错误)

template <typename T>
template <typename E>
Grid<T>::Grid(const Grid<E>& src)
{
    cout << "Copy constructor working " << endl;

}

错误:

Severity Code Description Project File Line Suppression State Error LNK2019 unresolved external symbol "public: __thiscall Grid::Grid(unsigned int,unsigned int)" (??0?$Grid@N@@QAE@II@Z) referenced in function _main

Error LNK1120 2 unresolved externals

Error LNK2019 unresolved external symbol "public: virtual __thiscall Grid::~Grid(void)" (??1?$Grid@N@@UAE@XZ) referenced in function "public: virtual void * __thiscall Grid::`scalar deleting destructor'(unsigned int)" (??_G?$Grid@N@@UAEPAXI@Z)

Answer 1

模板构造函数永远(!)不是复制构造函数。您代码中的构造函数只是一个转换构造函数。

你可能两者都想要:

#include <iostream>
#include <vector>

// Please do not have this ugliness in a header!
using namespace std;

template <typename T>
class Grid
{
public:
    explicit Grid(size_t inWidth = kDefaultWidth, size_t inHeight = kDefaultHeight)
    // initialize members ...
    {}

    // Without this copy-constructor the compiler generates a copy-constructor
    // (with no output, of course)
    Grid(const Grid& src) 
    // initialize members ...
    {
        cout << "Copy constructor" << endl;
    }

    template <typename E>
    Grid(const Grid<E>& src) 
    // initialize members ...
    {
        cout << "Converting constructor" << endl;
    }

    static const size_t kDefaultWidth = 10;
    static const size_t kDefaultHeight = 10;
    size_t mWidth, mHeight;
};

int main()
{
    Grid<double> myDoubleGrid;
    Grid<double> newDoubleGrid(myDoubleGrid); // Copy constructor
    Grid<int> newIntGrid(myDoubleGrid);       // Converting constructor
    return 0;
}

另见:

• http://en.cppreference.com/w/cpp/language/copy_constructor
• http://en.cppreference.com/w/cpp/language/converting_constructor
• Derive from template constuctor of template base class