有没有办法在编译时实例化所有c ++模板大小？

Question

下面的c ++/cli模板正在运行,但似乎应该有一种方法可以进一步概括模板或添加一个可以在编译时创建模板实例的帮助器.

想到像http://en.cppreference.com/w/cpp/utility/integer_sequence这样的东西可能有用,但需要帮助/实施者功能的一些帮助.

简化主要内容以演示所需语法与当前使用的语法:

int main(array<String^>^ args) {

    // the actual number of possible char lengths is sparse (generally)
    // but the API allows for 1-1024
    List<int>^ varList = gcnew List<int>();
    varList->Add(40);
    varList->Add(80);
    varList->Add(128);

    SortedList<int, List<String^>^>^ allStrings = gcnew SortedList<int, List<String^>^>();

    // want something like this, but the compiler complains that 
    // the template is invalid expectes compile-time constant expressions
    for each(int key in varList) {
        allStrings->Add(key, UpdateTest<key>());
    }

    // this works, but has 1024 lines of case N:
    for each(int key in varList) {
        switch (key) {
        case 1: allStrings->Add(key, UpdateTest<1>());
        case 2: allStrings->Add(key, UpdateTest<2>());
        case 3: allStrings->Add(key, UpdateTest<3>());
            //... all 1024 possible char[N] sizes...
        case 1024: allStrings->Add(key, UpdateTest<1024>());
        }
    }
}

模板适用于1024个交换机案例N:调用.有没有办法让一个帮助器/实现者实例化所有1024没有案例？

template <std::size_t N> List<String^>^ APIwrapper::UpdateTest() {
    typedef char CHARX[N];  // N valid for 1 to 1024
    CHARX vals[MAXFIELDS];

    // NATIVE C++ VendorAPI(short, void*) vals is the address of the word aligned destination data
    int count = VendorAPI(N, &vals); 

    List<String^>^ retList = gcnew List<String^>();

    for (int c = 0; c < count; c++) {
        CHARX temp;
        strncpy(temp, vals[c], N);  // \0 terminated char arrays
        String^ x = gcnew String(temp);
        retList->Add(x->Substring(0, N)->Trim());
    }
    return retList;
}

Answer 1

以下答案适用于c ++ 14,我不知道它是否与cli兼容.

您可以使用模板生成std::array函数指针:

using UpdateTestPtr = decltype(&UpdateTest<0>);

// Struct used to generate the array's content.
template<typename Sequence>
struct UpdateTestArrayImpl;

// Specialization used to get the values in the integer sequence.
template<std::size_t... indices>
struct UpdateTestArrayImpl<std::integer_sequence<std::size_t,indices...>> {
    static constexpr std::array<UpdateTestPtr,sizeof...(indices)> value{UpdateTest<indices>...};
};

// Factorise sequence creation
template<std::size_t N>
using UpdateTestArray = UpdateTestArrayImpl<std::make_index_sequence<N>>;

static constexpr std::size_t N = 512;
// The array is needed at runtime. Create a normal (not constexpr) instance.
static std::array<UpdateTestPtr,N> functionArray = UpdateTestArray<N>::value;

并将其switch/case转换为数组查找:

for each(int key in varList) {
    allStrings->Add(key, functionArray[key]());
}

现场演示

某些编译器可能会产生"模板实例化深度"错误,具体取决于make_integer_sequence实现方式.通常可以使用编译器选项来增加递归深度的最大限制.