[英]Can `throw` be inside a comma subexpression within C++ conditional (ternary) operator?
It is well known that throw
can be placed as the second or the third operand of C++ ternary operator ?:
.众所周知, throw
可以作为 C++ 三元运算符?:
的第二个或第三个操作数。 But can it be inside a comma subexpression of there operands?但是它可以在操作数的逗号子表达式中吗? It looks like compilers diverge in this regard.看起来编译器在这方面存在分歧。 Please consider an example:请考虑一个例子:
#include <iostream>
void foo(bool b) {
int i = b ? 1 : (throw 0); //ok everywhere
if ( !b )
(std::cout << "smth\n", throw 0); //ok everywhere
i = b ? 2 : (std::cout << "smth\n", throw 0); //ok in MSVC only
};
This example is accepted by MSVC, but rejected by both GCC and Clang, demo: https://gcc.godbolt.org/z/6q46j5exP这个例子被MSVC接受,但是被GCC和Clang都拒绝了,demo: https://gcc.godbolt.org/z/6q46j5exP
Though the error message:虽然错误信息:
error: third operand to the conditional operator is of type 'void', but the second operand is neither a throw-expression nor of type 'void'
7 | i = b ? 2 : (std::cout << "smth\n", throw 0);
| ^
suggests that it was not rejected intentionally but rather the compiler thinks that the third operand not only has formal type void
but actually can return.表明它不是有意拒绝的,而是编译器认为第三个操作数不仅具有正式类型void
,而且实际上可以返回。
According to https://en.cppreference.com/w/cpp/language/operator_other , it seems that GCC/Clang are right since根据https://en.cppreference.com/w/cpp/language/operator_other ,似乎 GCC/Clang 是正确的,因为
Either E2 or E3 (but not both) is a (possibly parenthesized) throw-expression. E2 或 E3(但不是两者)是一个(可能带括号的)抛出表达式。
and here we have parenthesized comma expression just finishing with throw-expression.在这里我们用括号括起来的逗号表达式以 throw-expression 结尾。
According to the standard, is MSVC incorrect in accepting the last line of the example?根据标准,MSVC 在接受示例的最后一行时是否不正确?
Clang and GCC are correct to reject it. Clang 和 GCC 拒绝它是正确的。 It's pretty straightforward:这很简单:
[expr.cond] [表达式条件]
2 If either the second or the third operand has type
void
, one of the following shall hold: 2如果第二个或第三个操作数的类型为void
,则应满足以下条件之一:
- The second or the third operand (but not both) is a (possibly parenthesized) throw-expression ( [expr.throw] );第二个或第三个操作数(但不是两者)是一个(可能带括号的)throw 表达式( [expr.throw] ); the result is of the type and value category of the other.结果属于另一个的类型和值类别。 The conditional-expression is a bit-field if that operand is a bit-field.如果该操作数是位域,则条件表达式是位域。
- Both the second and the third operands have type
void
;第二个和第三个操作数的类型都是void
; the result is of typevoid
and is a prvalue.结果的类型为void
,并且是纯右值。
The wording is pretty precise here.这里的措辞非常准确。 It says one operand is a throw-expression when the first bullet applies.它说当第一个项目符号适用时,一个操作数是一个抛出表达式。 And (std::cout << "smth\n", throw 0)
is not a throw-expression.并且(std::cout << "smth\n", throw 0)
不是抛出表达式。 It's parenthesized comma expression.它是带括号的逗号表达式。
So we can only be in the case of the second bullet, but its conditions don't hold either.所以我们只能在第二颗子弹的情况下,但它的条件也不成立。 So a "shall" requirement is broken, and the program is thus ill-formed.所以“应该”的要求被打破了,程序因此是病式的。
Now, MSVC may be offering an extension around this, but it's not standard.现在,MSVC 可能会为此提供扩展,但它不是标准的。
As noted by @StoryTeller - Unslander Monica , there is a specification restriction with operands evaluating to void
.正如@StoryTeller-Unslander Monica所指出的,对计算为void
的操作数存在规范限制。
I should note, however, that in this case the restriction is trivially bypassed:然而,我应该注意到,在这种情况下,限制被简单地绕过了:
#include <iostream>
void foo(bool b) {
int i = b ? 1 : (throw 0); //ok everywhere
if ( !b )
(std::cout << "smth\n", throw 0);
// OR
i = b ? 2 : throw ((std::cout << "smth\n"), 0);
// OR
i = b ? 2 : (std::cout << "smth\n", throw 0, 2);
// ^^^ extra addition
}
The extra , 2
changes the type of (std::cout << "smth\n", throw 0, 2)
to int
, at which point the quoted rule no longer applies -- the operand is no longer of type void
-- and therefore there is no longer any restriction on the operand. extra , 2
将(std::cout << "smth\n", throw 0, 2)
的类型更改为int
,此时引用的规则不再适用——操作数不再是void
类型——因此对操作数不再有任何限制。
(And now we can question what is the point of the restriction in the standard...) (现在我们可以质疑标准中限制的意义是什么......)
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