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为什么指定的初始值设定项没有在 g++ 中实现

发布于 2024-10-16 04:58:26 字数 95 浏览 4 评论 0原文

g++ 中没有添加对指定初始值设定项的支持有什么具体原因吗？是因为C99标准来得晚，g++发展得更早，后来人们不关心这个问题，还是C++语法中实现指定初始化器有一些固有的困难？

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牵你的手，一向走下去 2024-10-23 04:58:26

我今天遇到了同样的问题。 g++ 与 -std=c++11 和 c++14 确实支持指定的初始值设定项，但如果您不要按照定义结构成员的顺序初始化该结构。举个例子

struct x
{
    int a;
    int b;
};

// This is correct
struct x x_1 = {.a = 1, .b = 2};
// This will fail to compile with error non-trivial designated initializer
struct x x_2 = {.b = 1, .a = 2};

I ran into this same problem today. g++ with -std=c++11 and c++14 does support designated initializers, but you can still get a compilation error "test.cxx:78:9: sorry, unimplemented: non-trivial designated initializers not supported" if you don't initialize the struct in the order in which it's members have been defined. As an example

struct x
{
    int a;
    int b;
};

// This is correct
struct x x_1 = {.a = 1, .b = 2};
// This will fail to compile with error non-trivial designated initializer
struct x x_2 = {.b = 1, .a = 2};

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放赐 2024-10-23 04:58:26

正如我在评论中指出的，G++ 不支持 C99 标准指定初始化程序，但它确实支持 C90 的 GNU 扩展，允许指定初始化程序。所以这行不通：

union value_t {
    char * v_cp;
    float v_f;
};
union value_t my_val = { .v_f = 3.5f };

但这确实行得通：

union value_t my_val = { v_f: 3.5f };

这似乎是 C 和 C++ 标准委员会之间协调的不良互动（C++ 没有特别充分的理由为什么不支持 C99 语法，他们只是没有这样做）没有考虑过）和 GCC 政治（C++ 不应该仅仅因为它是 C99 语法就支持 C99 语法，但它应该支持实现完全相同的事情的 GNU 扩展语法，因为这是一个可以应用于任何一种语言的 GNU 扩展）。

As I noted in a comment, G++ doesn't support C99 standard designated initialisers, but it does support the GNU extension to C90 which allows designated initialisers. So this doesn't work:

union value_t {
    char * v_cp;
    float v_f;
};
union value_t my_val = { .v_f = 3.5f };

But this does:

union value_t my_val = { v_f: 3.5f };

This seems to be a bad interaction of co-ordination between the C and C++ standards committees (there is no particularly good reason why C++ doesn't support the C99 syntax, they just haven't considered it) and GCC politics (C++ shouldn't support C99 syntax just because it's in C99, but it should support GNU extension syntax that achieves exactly the same thing because that's a GNU extension that can be applied to either language).

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