为什么默认参数不能依赖非默认参数? [英] why can't default argument depend on non-default argument?

问题描述

考虑以下构造函数:

class MyClass {
    MyClass(unsigned int dimension, std::vector vector=unitaryVector(dimension));
};

其中 unitaryVector(d)是一个函数，该函数返回d维的随机 std :: vector .

where unitaryVector(d) is a function that returns a random std::vector in d dimensions.

这会导致以下编译器错误:

This gives the following compiler error:

error: default argument references parameter 'dimension'
    MyClass(unsigned int dimension, std::vector vector=unitaryVector(dimension));

为什么这个成语在C ++ 11中无效?这似乎很明显:如果提供了 vector 参数，则将 vector 用作参数的副本，否则，调用该函数并将其作为return的副本进行初始化价值.为什么编译器不能理解这一点?

Why isn't this idiom valid in C++11? It seems to be pretty obvious: if vector argument is provided, init vector as a copy of the argument, otherwise, call the function and init it as a copy of the return value. Why can't a compiler understand this?

推荐答案

C ++标准禁止使用它.

The C++ standard forbids it.

dcl.fct.default

9 默认参数每次调用该函数时都会被评估没有相应参数的参数.参数不得在默认参数中显示为可能评估的表达式.在默认参数之前声明的函数的参数位于范围，并且可以隐藏名称空间和类成员名称.

9 default argument is evaluated each time the function is called with no argument for the corresponding parameter. A parameter shall not appear as a potentially-evaluated expression in a default argument. Parameters of a function declared before a default argument are in scope and can hide namespace and class member names.

[示例:

int a;
int f(int a, int b = a);            // error: parameter a
                                    // used as default argument
typedef int I;
int g(float I, int b = I(2));       // error: parameter I found
int h(int a, int b = sizeof(a));    // OK, unevaluated operand

—结束示例]

请注意，如果未提供默认参数，则会在呼叫站点上替换

Note that default arguments are replaced at the call site if not provided

简介执行 (强调我的意思)

11: [注意:对 完整表达式 的评估可以包括对以下子表达式的评估:在词汇上不是的一部分完整表达.例如，涉及评估的子表达式默认参数([dcl.fct.default])被认为是创建的在调用函数的表达式中，而不是定义默认参数. —尾注]

11: [ Note: The evaluation of a full-expression can include the evaluation of subexpressions that are not lexically part of the full-expression. For example, subexpressions involved in evaluating default arguments ([dcl.fct.default]) are considered to be created in the expression that calls the function, not the expression that defines the default argument. — end note ]

---

您可以简单地重载构造函数并委托它:

---

You can simply overload the constructor and delegate it:

class MyClass {
    explicit MyClass(unsigned int dimension) 
        : MyClass(dimension, unitaryVector(dimension))  //delegation
    {  }
    MyClass(unsigned int dimension, std::vector vector);
};

脚注:使单参数构造函数 explicit

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