模板類聲明 - 不是一個常量表達式

Question

我有以下類，我需要有兩個常量聲明。

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而且 這裏是其使用的測試代碼

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

    // other code and declaration 

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


} 

我收到以下錯誤：

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

爲了實例化模板，編譯器必須知道在所有模板參數編譯時間。沒有辦法在編譯時找出pageDivSize和bufferNum的值。所以模板參數不應該是一個常量變量，而應該是一個常量表達式。

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