&（int）{1}在C ++中是什么意思？ [英] What does &(int) { 1 } mean in C++?

问题描述

我在此处看到了，但我不知道这意味着什么：

I saw this here and I don't know what it means:

&(int) { 1 }

我认为这很奇怪，因为它看起来像无效的语法。它会在中间插入一个随机数为1（无分号）的块范围（？），并获取地址。对我来说没有多大意义。你们能启发我吗？

I thought it was weird because it seems like invalid syntax. It's casting a block scope(?) with a random 1 in the middle (without a semi-colon) and taking the address. Doesn't make a lot of sense to me. Could you guys enlighten me?

使用C ++ 11进行了尝试，因此可以编译：

Tried it out w/ C++11, so it compiles:

auto a = &(int) { 1 };

但是我不知道该变量如何处理。

But I have no idea what to do with the variable.

推荐答案

据我所知，这是复合文字，它是C99功能，它是不是标准C ++ ，但两者gcc和clang支持它作为扩展：

As far as I can tell this is a compound literal, it is C99 feature, it is not standard C++ but both gcc and clang support it as an extension:

ISO C99支持复合文字。复合文字看起来像包含初始化程序的强制类型转换。它的值是强制类型转换中指定类型的对象，其中包含初始化程序中指定的元素；它是一个左值。作为扩展，GCC在C90模式和C ++中支持复合文字，尽管语义在C ++中有所不同。

ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue. As an extension, GCC supports compound literals in C90 mode and in C++, though the semantics are somewhat different in C++.

通常，指定的类型是结构。假设如下所示声明了struct foo和
结构：

Usually, the specified type is a structure. Assume that struct foo and structure are declared as shown:

 struct foo {int a; char b[2];} structure;

以下是使用复合
文字构造struct foo的示例：

Here is an example of constructing a struct foo with a compound literal:

 structure = ((struct foo) {x + y, 'a', 0});

这等效于编写以下内容：

This is equivalent to writing the following:

 {
   struct foo temp = {x + y, 'a', 0};
   struct

在这种情况下， a 将指向int。希望这最初是C代码，因为gcc文档说：

In this case the type of a would pointer to int. Hopefully this was originally C code since as the gcc document says:

在C中，复合文字用静态或自动存储来指定未命名的对象持续时间。在C ++中，复合文字表示一个临时对象，该对象仅在其完整表达式结束之前一直存在。

In C, a compound literal designates an unnamed object with static or automatic storage duration. In C++, a compound literal designates a temporary object, which only lives until the end of its full-expression.

在C ++中，address可能不是一个好主意，因为对象的生命周期在完整表达式的结尾就结束了。虽然，可能是C ++代码只是依赖未定义的行为。

and so taking the address in C++ is probably a bad idea since the lifetime of the object is over at the end of the full-expression. Although, it could have been C++ code which just relied on undefined behavior.

这是在gcc和gcc中使用正确标志确实有很大帮助的情况之一使用 -pedantic 发出的叮当声将产生警告和错误，例如gcc说：

This is one of those cases where using the correct flags really helps a lot, in both gcc and clang using -pedantic will produce a warning and an error, for example gcc says:

warning: ISO C++ forbids compound-literals [-Wpedantic]
 auto a = &(int) { 1 };
                     ^
error: taking address of temporary [-fpermissive]

的地址使用 -fpermissive 是gcc，它确实允许编译此代码。尽管旧版本似乎允许使用 -Wno-address-of-temporary ，但在现代版本中，我无法使用clang来构建带有任何标志的代码。我想知道gcc是否允许它作为扩展。

if we use -fpermissive is gcc it indeed does allow this code to compile. I can not get clang to build this code with any flags in modern versions although old versions seem to allow it using -Wno-address-of-temporary. I wonder if gcc allows this as a remnant of an old extension.

请注意，问题 C中的隐式结构定义使用复合文字非常有趣（取决于您对有趣的定义的使用）。

Note, the question Cryptic struct definition in C has a pretty interesting(depending on your definition of interesting) use of compound literals.

假定分裂是正确的和这个问题是原始来源，然后是示例中该代码中的用法：

Assuming Schism is correct and this question is the original source then the use in that code in the example:

if (setsockopt(server_connection.socket, SOL_SOCKET, SO_REUSEADDR, &(int) { 1 }, sizeof(int)) < 0) {

在C99和C ++中都是有效的用法。

is a valid use in both C99 and C++.

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