地图不完整的值类型

问题描述

我遇到以下错误：

  class Test 
 {
 std： ：地图<的std :: string，试验>测试; 
}; 
  

错误是Field has incomplete type'Test'。我读了几个线程，建议这可能是libcxx版本的xcode附带的错误，但是如果我只需要将其更改为：

  class Test 
 {
 std :: map< std :: string，std :: shared_ptr< Test>>测试; 
}; 
  

我只是想检查一下，这绝对是一个正确的错误，而不是一个错误。 >

Cheers！

解决方案

该标准要求，模板组件中使用的所有类型的标准库必须完整，除非另有说明。因此libc ++的错误是正确的。使用不完整的类型是未定义的行为（§17.6.4.8[res.on.functions] / 2），所以libstdc ++接受它也没有错。

您可以使用指针对象来构建完整的类型。但您也可以使用一些第三方集合库，这些库允许不完整的类型，例如是。

---

§17.6.4.8其他功能/ 2：

（明确允许不完整类型的唯一组件是 declare ， unique_ptr ， default_delete ， shared_ptr ， weak_ptr 和 enable_shared_from_this 。）

I'm getting an error with the following:

class Test
{
    std::map<std::string,Test> test;
};

The error is "Field has incomplete type 'Test'". I read a few threads with suggested this might be a bug in the version of libcxx which ships with xcode, but it wouldn't surprise me at all if I just have to change it to:

class Test
{
    std::map<std::string,std::shared_ptr<Test>> test;
};

I just wanted to double check that this is definitely a correct error and not a bug.

Cheers!

解决方案

The standard requires that, all types used in template components of the standard library must be complete, unless otherwise stated. Thus libc++'s error is correct. Using an incomplete type is undefined behavior (§17.6.4.8[res.on.functions]/2), so libstdc++ accepting it isn't wrong either.

You could use a pointer object to construct the complete type as you did. But you could also use some third-party collections library which allows incomplete types, e.g. boost::container::map is designed to allow incomplete types.

---

§17.6.4.8 Other functions / 2:

(The only components that explicitly allow incomplete types are declval, unique_ptr, default_delete, shared_ptr, weak_ptr and enable_shared_from_this.)

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