为什么不能将构造函数语法与"unsigned int"一起使用?类型? [英] Why can constructor syntax not be used with the "unsigned int" type?

问题描述

为什么以下内容在C ++中是非法的?

Why is the following illegal in C++?

auto x = unsigned int(0);

以下各项都可以:

auto y = int(0);
auto z = unsigned(0);
auto w = float(0);

或一般而言:

auto t = Type(... c-tor-args ...);

(Type是unsigned int除外).

推荐答案

语法为此处的显式类型转换(功能符号).根据语法规则，它仅适用于简单的类型说明符或typedef说明符(即单字类型名称).

The syntax is Explicit type conversion (functional notation) here. According to the grammatical rule, it only works with simple type specifier or typedef specifier (i.e. a single-word type name).

(重点是我的)

2)函数强制转换表达式由简单类型说明符或typedef说明符组成(换句话说，单个单词类型名称:unsigned int(expression)或int*(expression)无效)，后面加上括号中的单个表达式.该强制转换表达式与相应的C样式强制转换表达式完全等效.

2) The functional cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name: unsigned int(expression) or int*(expression) are not valid), followed by a single expression in parentheses. This cast expression is exactly equivalent to the corresponding C-style cast expression.

您可以将其更改为c样式强制转换表达式或static_cast，或将其与类型定义说明符一起使用，如@Jean-FrançoisFabre建议的那样.

You can change it to c-style cast expression or static_cast, or use it with typedef specifier as @Jean-FrançoisFabre suggested.

auto x1 = (unsigned int)(0);
auto x2 = static_cast<unsigned int>(0);

来自标准报价， $ 5.2.3/1显式类型转换(功能符号)[expr.type.conv]

简单类型说明符([dcl.type.simple])或类型名称说明符([temp.res])，后跟带括号的可选表达式列表或括号初始化列表(初始化程序)构造给定初始值设定项的指定类型的值.

A simple-type-specifier ([dcl.type.simple]) or typename-specifier ([temp.res]) followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer.

和 $ 7.1.7.2/1简单类型说明符[dcl.type.simple ]

简单类型说明符是

The simple type specifiers are

simple-type-specifier:
    nested-name-specifieropt type-name
    nested-name-specifier template simple-template-id
    nested-name-specifieropt template-name
    char
    char16_t
    char32_t
    wchar_t
    bool
    short
    int
    long
    signed
    unsigned
    float
    double
    void
    auto
    decltype-specifier
type-name:
    class-name
    enum-name
    typedef-name
    simple-template-id
decltype-specifier:
  decltype ( expression )
  decltype ( auto )

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