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经过多年的缺席,我将回到 C++(或者从技术上讲,Objective-C++),所以请多多包涵。我正在尝试使用模板来实现一个解决方案,否则它需要大量的剪切和粘贴代码。下面的摘录显示了这项工作的基本要点:

namespace {

    using vMAT::DOUBLE;
    using vMAT::SINGLE;
    using vMAT::INT8;
    using vMAT::UINT8;
    using vMAT::INT16;
    using vMAT::UINT16;
    using vMAT::INT32;
    using vMAT::UINT32;
    using vMAT::INT64;
    using vMAT::UINT64;

    typedef void (* swapFn)(void * vector, vDSP_Length vectorLength);

    // Functor used for template arguments.
    template <typename TypeA>
    struct swapbytes {
        swapFn fn;

        swapbytes()
        {
            if      (sizeof(TypeA) == 8) fn = vMAT_byteswap64;
            else if (sizeof(TypeA) == 4) fn = vMAT_byteswap32;
            else if (sizeof(TypeA) == 2) fn = vMAT_byteswap16;
            else if (sizeof(TypeA) == 1) fn = NULL;
            else NSCAssert(NO, @"Oops!");
        }

        void operator()(void * vector, vDSP_Length vectorLength) const
        // ..................................................... ^^^^^ Why?
        {
            if (fn != NULL) fn(vector, vectorLength);
        }
    };

    template <typename TypeA, typename ClassB>
    void
    loadFromOperation(vMAT_MATv5NumericArray * self,
                      vMAT_MATv5ReadOperation * operation,
                      TypeA a,
                      ClassB b)
    {
        swapbytes<TypeA> SwapA;
        long lenC = self.size[0] * sizeof(TypeA);
        TypeA * C = (TypeA *)malloc(lenC);
        long lenD = vMAT_Size_prod(self.size) * sizeof(ClassB);
        self.arrayData = [NSMutableData dataWithCapacity:lenD];
        self.arrayData.length = lenD;
        ClassB * D = (ClassB *)[self.arrayData mutableBytes];
        __block long idxD = 0;
        vMAT_Size123Iterator(self.size, ^(int32_t n, int32_t o, int32_t p) {
            [operation readComplete:C
                             length:lenC];
            if (operation.swapBytes) { SwapA((void *)C, lenC / sizeof(TypeA)); }
            for (int m = 0;
                 m < self.size[0];
                 m++) {
                D[idxD] = C[m];
                ++idxD;
            }
        });
        free(C);
    }

}

- (void)_load_miUINT8_mxDOUBLE_fromOperation:(vMAT_MATv5ReadOperation *)operation;
{
    loadFromOperation(self, operation, UINT8, DOUBLE);
}

我希望从以“为什么?”结尾的评论中可以看出我的问题是,为什么const这里需要声明?如果省略,编译器会抱怨SwapA(...)loadFromOperation.

SwapA如果明确声明为,我会理解这一点const,但事实并非如此。显然它可能是,因为它的操作不依赖于改变任何状态,但这不是重点。

虽然我对这段代码有另一双有用的眼睛,但我欢迎更精通 C++ 实践者可能愿意提供的任何风格或其他评论……</p>

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1 回答 1

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由块捕获的非__block变量,在创建块时按值复制到块中,并且const位于块内。即在块内,SwapA具有类型const swapbytes<TypeA>.

于 2013-04-04T22:18:03.737 回答