VS2019/2022中前置声明下variant与vector编译问题咨询

问题 #

在Visual Studio 2019或2022中，以下代码里变量a和b可以正常编译，但变量c无法编译，报错信息如下，请问原因是什么？有哪些解决办法？

#include <variant>
#include <vector>

struct data; // 定义在另一文件中

std::variant<data>* a; // 正常
std::vector<data*>  b; // 正常

std::vector<std::variant<data>*> c; // 错误 C2139: 'data': 未定义的类不能作为编译器固有类型特性'__is_trivially_destructible'的参数

完整错误信息：

1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(931,60): error C2139: 'data': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_trivially_destructible'
1>C:\Temp\test2019\main.cpp(4): message : see declaration of 'data'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(931): message : see reference to variable template 'const bool conjunction_v<std::is_trivially_destructible<data> >' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(1006): message : see reference to alias template instantiation 'std::_Variant_destroy_layer<data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\vector(1667): message : see reference to class template instantiation 'std::variant<data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\vector(1665): message : while compiling class template member function 'void std::vector<std::variant<data> *,std::allocator<std::variant<data> *>>::_Destroy(std::variant<data> **,std::variant<data> **)'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\vector(1751): message : see reference to function template instantiation 'void std::vector<std::variant<data> *,std::allocator<std::variant<data> *>>::_Destroy(std::variant<data> **,std::variant<data> **)' being compiled
1>C:\Temp\test2019\main.cpp(8): message : see reference to class template instantiation 'std::vector<std::variant<data> *,std::allocator<std::variant<data> *>>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xsmf_control.h(60,19): error C2139: 'data': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_constructible'
1>C:\Temp\test2019\main.cpp(4): message : see declaration of 'data'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xsmf_control.h(60): message : see reference to variable template 'const bool conjunction_v<std::is_move_constructible<data>,std::negation<std::conjunction<std::is_trivially_move_constructible<data> > > >' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xsmf_control.h(97): message : see reference to alias template instantiation 'std::_SMF_control_move<std::_Variant_destroy_layer_<data>,data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xsmf_control.h(39,19): error C2139: 'data': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_constructible'
1>C:\Temp\test2019\main.cpp(4): message : see declaration of 'data'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xsmf_control.h(39): message : see reference to variable template 'const bool conjunction_v<std::is_copy_constructible<data>,std::negation<std::conjunction<std::is_trivially_copy_constructible<data> > > >' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xsmf_control.h(61): message : see reference to alias template instantiation 'std::_SMF_control_copy<std::_Variant_destroy_layer_<data>,data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(462,32): error C2079: 'std::_Variant_storage_<false,data>::_Head' uses undefined struct 'data'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(810): message : see reference to class template instantiation 'std::_Variant_storage_<false,data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(916): message : see reference to class template instantiation 'std::_Variant_base<data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xsmf_control.h(82): message : see reference to class template instantiation 'std::_Variant_destroy_layer_<data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\xsmf_control.h(120): message : see reference to class template instantiation 'std::_Deleted_copy_assign<std::_Variant_destroy_layer_<data>,data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(1006): message : see reference to class template instantiation 'std::_Deleted_move_assign<std::_Variant_destroy_layer_<data>,data>' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(1008,86): error C2139: 'data': an undefined class is not allowed as an argument to compiler intrinsic type trait '__is_destructible'
1>C:\Temp\test2019\main.cpp(4): message : see declaration of 'data'
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(1008): message : see reference to variable template 'const bool conjunction_v<std::is_object<data>,std::negation<std::is_array<data> >,std::is_destructible<data> >' being compiled
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.29.30133\include\variant(1008,19): error C2338: variant<Types...> requires all of the Types to meet the Cpp17Destructible requirements N4828 [variant.variant]/2.

原因分析 #

• 变量a编译正常：std::variant<data>*是指针类型，C++允许用不完整类型（仅前向声明）声明指向它的指针，因为指针大小固定，编译器不需要知道data的具体结构就能处理该指针类型。
• 变量b编译正常：std::vector<data*>存储的是data类型的指针，vector的核心操作（如内存分配、指针存储）仅依赖指针本身，不需要data的完整定义，前向声明足够。
• 变量c编译失败：要实例化std::vector<std::variant<data>*>，编译器必须先实例化std::variant<data>模板。而MSVC的std::variant实现会在实例化阶段检查模板参数的类型特性（如是否可平凡析构、是否可析构等），这些内置类型特性要求参数必须是完整类型。但data仅为前向声明，属于不完整类型，无法满足检查要求，因此触发编译错误。从错误信息可看出，编译器处理variant的销毁层、存储层逻辑时，需要访问data的构造、析构特性，而这些都依赖data的完整定义。

解决办法 #

1. 提供data的完整定义 #

在使用std::variant<data>之前，包含定义data的头文件，让编译器获取data的完整结构。这是最直接的方案，也符合C++17标准对std::variant的要求（标准明确规定variant的所有模板参数必须是可析构的完整类型）。

2. 用封装类延迟variant实例化 #

如果无法提前获取data的完整定义，可以引入中间封装类，通过前向声明延迟std::variant<data>的实例化：

#include <vector>

struct VariantData; // 前向声明
std::vector<VariantData*> c;

// 在data完整定义的文件中实现VariantData
#include "data.h"
#include <variant>
struct VariantData {
    std::variant<data>* var_ptr;
    // 按需添加构造、析构等逻辑
};

这种方式将variant的实例化推迟到data完整定义之后，避免编译时的类型检查错误。

内容的提问来源于stack exchange，提问作者Stéphane Chazel