c++ - std::vector 初始化的模板参数

Question

有一个名为 Matrix 的结构，它有一个模板参数 N 和 data_ 字段:

#include <cstddef>
#include <vector>

template <std::size_t N>
struct Matrix {
    std::vector<std::vector<int>> data_{N, std::vector<int>(N)};
};

为什么不能使用圆括号初始化data_？

std::vector<std::vector<int>> data_(N, std::vector<int>(N));

这是错误:

<source>:6:41: error: unknown type name 'N'
    std::vector<std::vector<int>> data_(N, std::vector<int>(N));
                                        ^
<source>:6:61: error: declaration of 'N' shadows template parameter
    std::vector<std::vector<int>> data_(N, std::vector<int>(N));
                                                            ^
<source>:4:23: note: template parameter is declared here
template <std::size_t N>

Answer 1

Default member initializer (C++11 起)不支持圆括号初始化器，只支持花括号初始化器和等号初始化器。

Through a default member initializer, which is a brace or equals initializer included in the member declaration and is used if the member is omitted from the member initializer list of a constructor.

除了你已经展示的支撑之外，你还可以

template <std::size_t N>
struct Matrix {
    std::vector<std::vector<int>> data_ = std::vector<std::vector<int>>(N, std::vector<int>(N));
};