[英]how to get the member in C++ struct initialization
struct Test {
int w, h;
int * p;
};
int main(){
Test t {
10,
20,
new int[this->h*this->w]
};
return 0;
}
I just want to use the w and h in initialization, is there any way to get this?我只想在初始化中使用 w 和 h,有什么办法可以得到这个吗?
First of all - you should avoid calling new
(and delete
) explicitly , except in rare case;首先 - 你应该避免显式调用new
(和delete
) ,除非在极少数情况下; this isn't one of them.这不是其中之一。 Use an std::unique_ptr
to hold your allocated memory (see below).使用std::unique_ptr
来保存您分配的 memory (见下文)。
To answer your question: You can't use the members of a struct/class as arguments to a constructor of that struct/class.要回答您的问题:您不能将结构/类的成员用作该结构/类的构造函数的 arguments。 Conceptually, the arguments are resolved before the constructor runs.从概念上讲,arguments 在构造函数运行之前被解析。
However, you can write a named constructor idiom:但是,您可以编写命名构造函数:
struct Test {
int w, h;
std::unique_ptr<int[]> p;
static:
Test make(int w, int h) {
return Test{ w, h, std::make_unique<int[]>(w*h) };
}
};
which would let you write:这会让你写:
auto my_test = Test::make(w, h);
Alternatively, you could just outright implement a constructor which takes only w
and h
:或者,您可以直接实现一个只需要w
和h
的构造函数:
struct Test {
int w, h;
std::unique_ptr<int[]> p;
Test(int w_, int h_) : w(w_), h(_), p(std::make_unique<int[]>(w_*h_) { }
};
... but then you would need to write some extra code for a no-parameter constructor and a 3-parameter constructor (if not other methods). ...但是您需要为无参数构造函数和 3 参数构造函数(如果不是其他方法)编写一些额外的代码。
If you write a constructor for your class, you can take advantage of its member initializer list .如果您为 class 编写构造函数,则可以利用其成员初始化列表。 In particular, you could exploit the fact that "non-static data member are initialized in order of declaration in the class definition" .特别是,您可以利用“非静态数据成员按 class 定义中的声明顺序初始化”这一事实。
Consider this little less trivial example考虑这个不那么琐碎的例子
#include <iostream>
#include <stdexcept>
#include <vector>
class Matrix
{
int h_{};
int w_{};
std::vector<int> d_;
public:
Matrix() = default;
Matrix(int h, int w)
: h_{checked_positive(h)}
, w_{checked_positive(w)}
, d_(h_ * w_) // <--
{}
void show() {
std::cout << h_ << ' ' << w_ << ' ' << d_.size() << '\n';
}
private:
int checked_positive(int d) {
if (d < 1)
throw std::runtime_error{"Dimensions must be positive"};
return d;
}
};
int main()
{
Matrix a(3, 4);
a.show();
}
Note, though, that some reviewers might find this dependency on the order of declaration of the members an unnecessary one and a maintainability cost.但请注意,一些审阅者可能会发现这种对成员声明顺序的依赖是不必要的,并且会增加可维护性成本。
In alternative, the dependent member could be default-initialized and then modified in the body of the constructor:或者,依赖成员可以默认初始化,然后在构造函数的主体中修改:
class Matrix
{
std::vector<int> d_; // <--
int h_{}, w_{};
public:
Matrix() = default;
Matrix(int h, int w)
: h_{checked_positive(h)}
, w_{checked_positive(w)}
{
d_.resize(h_ * w_); // <--
}
// ...
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