当所有逗号运算符不作为逗号运算符？ [英] When all does comma operator not act as a comma operator?

问题描述

如果您看到此代码，

class A{
public:
    A(int a):var(a){}
    int var;
};

int f(A obj) {
    return obj.var;
}

int main() {
    //std::cout<<f(23);    // output: 23
    std::cout<<f(23, 23);  // error: too many arguments to function 'int f(A)'
    return 0;
}

f（23，23）不编译，因为逗号作为分隔符而不是逗号运算符。

f(23, 23) does not compile because the comma acts as a separator here and not as a comma operator.

所有逗号不作为逗号运算符？

Where all does a comma not work as a comma operator? Or the other way around?

推荐答案

从语法的角度看，函数调用的参数形成一个可选的表达式列表在括号中。一个表达式列表由一个或多个赋值表达式组成，以逗号分隔。逗号只能表示期望使用表达式的逗号运算符。

From a grammatical point of view, the parameters of a function call form an optional expression-list inside parentheses. An expression-list consists of one or more assignment-expression separated by a comma token. A comma can only signify a comma operator where an expression is expected.

逗号运算符会生成 ，，和赋值语句一个逗号运算符本身不是一个 assignment-expression ，因此不能出现在 expression-list 中，除非它是一个

The comma operator makes an expression out of an expression, a , and an assignment-expression, but an expression involving a comma operator is not itself an assignment-expression so can't appear in an expression-list except where it's a constituent of something that is an assignment-expression.

例如，您可以使用括号中的任何表达式（包括使用逗号运算符的表达式） > primary-expression ，这是一个赋值语句，因此在表达式列表中有效。

For example, you can surround any expression (including one using the comma operator) inside parentheses to from a primary-expression which is an assignment-expression and hence valid in an expression-list.

Eg

postfix-expression 其中 expression-list / em>每个都是标识符。

postfix-expression where the expression-list consists of two assignment-expression each of which is an identifier.

f( a, b );

postfix-expression 其中 expression-list 由一个赋值表达式组成，它是一个使用逗号运算符的括号表达式的 primary-expression 。

postfix-expression where the expression-list consists of a single assignment-expression which is a primary-expression which is a parenthesized expression using the comma operator.

f( (a, b) );

这篇关于当所有逗号运算符不作为逗号运算符？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！