模板类声明 - 不是常量表达式

Question

我有以下类，我需要在声明时有两个常量。

template <int PAGE_DIV_SIZE, int BUFFERS_NUM>
    class BufferPool {
    //...
    }

和 这是一个使用它的测试代码

void testBufferPool(const int pageDivSize, const int bufferNum){   

    // other code and declaration

    BufferPool <pageDivSize, bufferNum> bufferPool(catalog, devNum, hostCapacityVec, devCapacityVec);


} 

我收到以下错误：

error: ‘pageDivSize’ is not a constant expression
BufferPoolTest.cpp:26:39: note: in template argument for type ‘int’ 
BufferPoolTest.cpp:26:39: error: ‘bufferNum’ is not a constant expression
BufferPoolTest.cpp:26:39: note: in template argument for type ‘int’ 
BufferPoolTest.cpp:26:51: error: invalid type in declaration before ‘(’ token
BufferPoolTest.cpp:26:100: error: expression list treated as compound expression in initializer [-fpermissive]
BufferPoolTest.cpp:26:100: error: cannot convert ‘std::vector<long unsigned int>’ to ‘int’ in initialization

Answer 1

为了实例化模板，编译器必须在编译时知道所有模板参数。在编译时无法找出pageDivSize和bufferNum的值。因此模板参数不应该是常量变量，而应该是常量表达式。

http://en.cppreference.com/w/cpp/language/constant_expression