如何将数组维度作为模板函数的参数传递 [英] How to pass array dimensions as parameters of a template function

问题描述

我需要一个函数来处理可变大小的二维char数组.以下是我的尝试之一:

I need a function to process a two-dimensional char array of variable sizes. Below is one of my attempts:

int GetCodes( char **codes, size_t N );
template<size_t N>
inline int GetCodes( char (*codes)[N] ){ return GetCodes( codes, N ); };

编译器(VC ++ 2008)抱怨说，它无法将参数1从" char(*)[8]"转换为"char **".所以我将代码更改为:

The compiler (VC++ 2008) complains that it "cannot convert parameter 1 from ''char (*)[8]'' to ''char **''." So I changed the code to:

int GetCodes( char **codes, size_t N );
template<size_t N>
inline int GetCodes( char (*codes)[N] ){ return GetCodes( (char**)codes, N ); };

那将编译并链接.但是，当它运行时，我将得到一个严重的指针错误，并且程序将崩溃.我该如何解决该问题?有没有不用模板就可以实现这一目标的方法?

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非常感谢所有建议，但我想补充一些说明.首先，我的问题是多维数组(在这种情况下为两个).我已经使用了Phillipe建议的一维数组参考，没有任何问题.我只是不能将其扩展到二维.我确实尝试使用引用，例如，

That would compile and link. However when it is run I would get a bad pointer error and the program would crash. How can I fix the problem? Is there a way to achieve this without using templates?

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Thanks so much for all the suggestions, but I would like to add a few clarifications. First of all my problem is with multi-dimensional (two in this case) arrays. I have used references for one-dimensional arrays as Phillipe suggested without any problem. I just cannot extend that to two-dimensions. I did try using references such as,

template<size_t N1, size_t N2>>
inline int GetCodes( char (&codes)[N1][N2] ){...};

但这有同样的问题.此外，我不需要第一个维度.以下是更完整的代码以显示预期的用途.

But it has the same problem. Besides, I don''t need the first dimension. Below is a more complete code to show the intended use.

int GetCodes( char **codes, const size_t N );
template<size_t N>
inline int GetCodes( char (*codes)[N] ){ return GetCodes( codes, N ); };

int GetCodes( char **codes, const size_t N )
{
	**codes = 'A';
	return 1;
}

int main()
{
	char codes[100][8];
	GetCodes( codes );
	return 0;
}

请注意，编译器将函数调用解析为模板函数没有问题.当模板函数尝试调用函数的两参数形式时，发生无法将char(*)[8]转换为char **"错误. Phillipe建议使用"char *(* codes)[N]"作为模板函数的参数.

正如Phillipie所说，在这种情况下，我想使用模板函数.如果那不可能，我将按照Barneyman的建议使用向量.

Please note that the compiler does not have problem resolving the function call to the template function. The "cannot convert char (*)[8] to char **" error occurs when the template function tries to call the two-parameter form of the function. Phillipe''s suggestion of using "char *(*codes)[N]" as the argument of the template function would not work.

As Phillipie said, I''d like to use template functions in this case. If that''s not possible I will use vectors as Barneyman suggested.

推荐答案

假设您要使用如下所示的数据调用函数:

Assuming that you want to call the function with data that look like that:

char* aa[] = { "AA", "BB", "CC", "DD" };
GetCodes(aa);

并且您希望N由编译器自动确定，那么您需要在声明中具有引用并修复括号.

And you want N to be automatically determined by the compiler, then you need to have a reference in the declaration and fix the parenthesis.

template<size_t N>
inline int GetCodes( char *(&codes)[N] ){ return GetCodes( codes, N ); };

需要该引用，以避免自动发生从数组到指针的转换.

例如:

The reference is required to avoid the automatic convertion from array to pointers that would otherwise occurs.

As an example:

template <typename T>
void Test1(T t)
{
    std::cout << "Test 1: " << typeid(t).name() << std::endl;
}

template <typename T>
void Test2(T &t)
{
    std::cout << "Test 2: " << typeid(t).name() << std::endl;
}

int main()
{
    int data[] = { 1, 2, 3, 4 };
    Test1(data);
    Test2(data);

    return 0;
}

这将提供以下输入:

This will give the following input:

Test 1: int *
Test 2: int [4]

我更倾向于只使用STL容器-向量或类似的容器

I''d be more inclined to just use an STL container - vector or similar

为了补充最初的解决方案，并作为对其他解决方案和评论的回应，我提供了一个将二维数组重新解释为一维数组的解决方案.

此代码主要用于教育目的.对于现实生活中的程序，通常建议使用STL或boost.

在该解决方案中，我添加了将所有项设置为模的全局代码的代码(如果我们将数组假装为一维，则为索引).

我还提供了使用引用或指针来显示差异的替代方法.

To complement my initial solution and as a response for other solutions and comment, I have provided a solution that reinterpret a two dimensional array as a single dimensional array.

This code is mainly for educational purpose. For real life program, using STL or boost would generally be recommanded.

Well in that solution, I have added code that will set each item to it global index modulo (the index if we would pretend the array to be single dimension).

I have also provides alternative using references or pointers to show the differences.

//void GetCodes(char *data, int count) // would works the same as next line
void GetCodes(char data[], int count)
{
    for (int i = 0; i != count; ++i)
    {
        data[i] = static_cast<char>(i %256);
    }
}

template<size_t M, size_t N>
void GetCodes(char (&data)[M][N])
{
    // Any of these produce the same result.
    GetCodes(reinterpret_cast<char(&)[M * N]>(data), M * N);
    //GetCodes(*reinterpret_cast<char(*)[M * N]>(data), M * N);
}

template<size_t M, size_t N>
void GetCodes(char (*data)[M][N])
{
    // Any of these produce the same result.
    GetCodes(reinterpret_cast<char(&)[M * N]>(*data), M * N);
    //GetCodes(*reinterpret_cast<char(*)[M * N]>(data), M * N);
    //GetCodes(*reinterpret_cast<char(*)[M * N]>(*data), M * N);
}

int main()
{
    char array1[100][8] = { 0 };
    char array2[100][8] = { 0 };

    GetCodes(array1);   // Calls first template
    GetCodes(&array2);  // Calls second template

    return 0;
}

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