默认参数使用另一个参数 [英] Default parameter using another parameter

问题描述

以下代码在C ++中无效

The following code is not valid in C++

struct Test {
    int x;
    int y;
};

void function(Test A, int n = A.x) {
    ...
}

因为默认参数Ax取决于A.有办法解决这个限制吗？我想这样做的原因如下。我有一个类Vector，行为非常接近std :: vector但有一个成员allocator_，它负责分配内存。我的拷贝构造函数有两个参数：

because the default parameter A.x depends upon A. Is there a way to go around this limitation? The reason why I want to do this is the following. I have a class Vector that behaves very closely to std::vector but has a member allocator_ which is responsible for allocating memory. My copy constructor takes 2 parameters:

Vector(const Vector& A, Allocator& allocator) {
    ...
}

如果第二个参数具有默认值， 。但我想分配器的默认值是A.allocator_这就是为什么我试过

Such copy constructors are allowed by the standard if the second parameter has a default value. But I want the default value for allocator to be A.allocator_ which is why I've tried

Vector(const Vector& A, Allocator& allocator = A.allocator_) {
    ...
}

不幸的是，它是无效的C ++。您是否有任何一个人有这个问题的解决方案？

Unfortunately, it is not valid C++. Does any one of you have a solution for this problem?

推荐答案

最简单的解决方案是使用重载而不是默认参数：

The easiest solution would be to use an overload instead of default arguments:

void function(Test A, int n) {
    ...
}

void function(Test A) {
    function(A, A.x);
}

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