Question

我正在写一个.csv导入器，它接收一个文件名并输出一个2d数组。

 template<size_t ROW,size_t COL>
 Matrix<double, ROW, COL > importcsv(string filename){
    ifstream myfile (filename);  //Constructs a stream, and then asssociates the stream with the      file "filename"
    vector<double> contentsasvector; // Vector which will store the contents of the stream.
    int i=1; int j=1;
    while(!myfile.eof()){
        if(myfile.get()==','){++j;}
        else if(myfile.get()=='\n'){++i;}
        else{contentsasvector.push_back(myfile.get());}
    }

myfile.close(); // Closes the filestream "my file"
constexpr int rows=i;
constexpr int cols=j/i;
Matrix<double, rows, cols> contents; // Array which will become a 2d version of the vector.
    for (int k = 0; k < rows; ++k){
        for (int l = 0; l < cols; ++l){
            contents[k][l]=contentsasvector[k*(j/i)+l];
        }
    }
    return contents;
}

令我感到困惑的是，我收到以下错误消息，即使我将行和列声明为constexpr。反正有吗？

error: non-type template argument is not a constant expression
    Matrix<double, rows, cols> contents; // Array which will become a 2d version of the vector.
                   ^~~~

Answer 1

您正在为行分配非常量值，这意味着它的值在编译时是未知的，因此您不能将其用作模板参数。编译器应该告诉你这个。如果您要在运行时选择行号和列号，我不确定模板是否是最好的选择。

如何转换为常量表达式

1 个答案: