Pass an array (variable) as template argument

Question

typedef int INT5[5] ;
    template<INT5 i5>
    void fun()
    {
        for (auto i : i5)
        {
            cout << i << endl;
        }
    }

    void test()
    {
        int i5[5] = { 2,3,1,2,4 };
        fun<i5>();//error
    }

I want to pass an array as a template argument, but failed.

error C2672: '__ExplicitVarTypeTest::fun': no matching overloaded function found

error C2971: '__ExplicitVarTypeTest::fun': template parameter 'i5': 'i5': a variable with non-static storage duration cannot be used as a non-type argument

Someone give me a solution to pass the varible as a fun's argument rather than the template's argument. But in my situation, I can't change the function's argument. because it's a callback function.

Actually, I need to do this

typedef char MyCharMap[UINT8_MAX];
    template<MyCharMap keymap>
    LRESULT CALLBACK LowLevelKeyboardProc(int nCode, WPARAM wParam, LPARAM lParam)
    {
        //MSDN说了，nCode<0（HC_ACTION）的都用CallNextHookEx返回
        if (nCode >= HC_ACTION)
        {
            KBDLLHOOKSTRUCT* pStruct = (KBDLLHOOKSTRUCT*)lParam;
            if (keyMap[pStruct->vkCode])
            {
                switch (wParam)
                {
                case WM_KEYDOWN:
                    keybd_event(keyMap[pStruct->vkCode], 0, 0, 0);
                    break;
                case WM_KEYUP:
                    keybd_event(keyMap[pStruct->vkCode], 0, KEYEVENTF_KEYUP, 0);
                    break;
                default:
                    throw "other key event";
                }
            }
            return TRUE;
        }
        //传给系统中的下一个钩子   
        return CallNextHookEx(keyHook, nCode, wParam, lParam);
    }

SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc<keymap1>, theApp.m_hInstance, NULL);
SetWindowsHookEx(WH_KEYBOARD_LL, LowLevelKeyboardProc<keymap2>, theApp.m_hInstance, NULL);

someone don't believe template can pass variable. just try template<int i> void fun(){cout<<i<<endl;} — Zhang
– Zhang, Commented Jun 8, 2018 at 6:26
@SauravSahu. That is not a duplicate. That questions asks how to pass an array as function argument. This question asks how to use an array as a non-type template parameter. — R Sahu
– R Sahu, Commented Jun 8, 2018 at 6:34
I reopened this question because the proposed duplicate doesn't address using an array as a non-type template argument. — Brian Bi
– Brian Bi, Commented Jun 8, 2018 at 6:34
@Brian, thank you, In my situation, I can't pass the array to the function as a argument, because it's a callback function. the MS defined it. — Zhang
– Zhang, Commented Jun 8, 2018 at 6:36

Brian Bi · Accepted Answer · 2018-06-08 06:39:57Z

22

Declaring a template to have a template parameter of type int[5] actually causes it to have a template parameter of type int*. This is similar to the decay that occurs in function signatures. To fix this, simply declare fun to take its array template argument by reference instead:

template<const INT5& i5> void fun();

You want to pass the array {2, 3, 1, 2, 4} as the template argument to fun. That's doable, but it has to obey the restrictions on glvalue constant expressions in [expr.const]/5 (C++17); it must refer to an object with static storage duration. Therefore, you should make i5 static:

static constexpr int i5[5] = { 2,3,1,2,4 };
fun<i5>();  // this should work

answered Jun 8, 2018 at 6:39

Brian Bi

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