如何使用具有匿名结构的匿名C++联合

Question

我需要语法方面的帮助，以访问结构中的联合，如下所示。编译器抱怨说，我需要在下面命名'innerStruct‘定义，而不是使用匿名内部结构。请有人解释一下规则，以及我如何初始化构造函数字段并命名位字段元素。我有一个coliru现场演示来显示代码。

不幸的是，代码没有编译，因为它指示了以下错误：

g++ -std=c++14 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
main.cpp: In constructor 'Foo::Foo(uint8_t, uint8_t)':
main.cpp:38:11: error: class 'Foo' does not have any field named 'asUint8'
         , asUint8(aBitFields)
           ^
main.cpp: In function 'std::ostream& operator<<(std::ostream&, const Foo&)':
main.cpp:54:83: error: 'const struct Foo' has no member named 'innerStruct'
             << "], mA[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mA
                                                                                   ^
main.cpp:55:83: error: 'const struct Foo' has no member named 'innerStruct'
             << "], mB[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mB
                                                                                   ^
main.cpp:56:83: error: 'const struct Foo' has no member named 'innerStruct'
             << "], mC[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mC
                                                                                   ^
main.cpp:57:83: error: 'const struct Foo' has no member named 'innerStruct'
             << "], mD[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mD
                                                                                   ^

struct Foo {
    // fields
    uint8_t mType;
    union innerUnion_t {
        struct innerStruct_t {
            uint8_t mA : 2;
            uint8_t mB : 1;
            uint8_t mC : 2;
            uint8_t mD : 3;
        } innerStruct;
        uint8_t asUint8;
    } innerUnion;

    // constructor
    explicit Foo() = default;

    // constructor
    explicit Foo(
        const uint8_t aType,
        const uint8_t aBitFields)
        : mType(aType)
        , asUint8(aBitFields)
    {}

    /**
     * Stream insert operator<p>
     *
     * @param os     [in,out] output stream
     * @param rhs    [in] Foo to send to the output
     *               stream.
     *
     * @return a reference to the updated stream
     */
    friend std::ostream& operator<<(
        std::ostream& os, const Foo& rhs) {
        os  << "Foo"
            << ": type[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.mType
            << "], mA[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mA
            << "], mB[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mB
            << "], mC[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mC
            << "], mD[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mD
            << "]";
        return os;
    }
};

编辑

注意:在建议的答案之后，对struct定义稍微做了一些修改，我还有一个关于初始化或引用位字段mA -> mD的语法的附加问题，我尝试在构造函数中使用以下语法，但它也没有编译。我试着将innerUnion命名为innerUnion，并明确地将其命名为嵌套结构为第二次现场演示

它给出了以下错误：

main.cpp: In constructor 'Foo::Foo(uint8_t, uint8_t)':
main.cpp:39:21: error: expected '(' before '.' token
         , innerUnion.innerStruct.mA(1)
                     ^
main.cpp:39:21: error: expected '{' before '.' token

Answer 1

您只需要删除代码中的一些单词：

#include <memory>
#include <iomanip>
#include <iostream>
#include <string>
#include <vector>

template<typename T>
std::ostream& operator<<(std::ostream& os, const std::vector<T>& vec)
{
    for (auto& el : vec)
    {
        os << el << ' ';
    }
    return os;
}

struct Foo {
    // fields
    uint8_t mType;
    union {
        struct {
            uint8_t mA : 2;
            uint8_t mB : 1;
            uint8_t mC : 2;
            uint8_t mD : 3;
        } innerStruct;
        uint8_t asUint8;
    };

    // constructor
    explicit Foo() = default;

    // constructor
    explicit Foo(
        const uint8_t aType,
        const uint8_t aBitFields)
        : mType(aType)
        , asUint8(aBitFields)
    {}

    /**
     * Stream insert operator<p>
     *
     * @param os     [in,out] output stream
     * @param rhs    [in] Foo to send to the output
     *               stream.
     *
     * @return a reference to the updated stream
     */
    friend std::ostream& operator<<(
        std::ostream& os, const Foo& rhs) {
        os  << "Foo"
            << ": type[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.mType
            << "], mA[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mA
            << "], mB[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mB
            << "], mC[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mC
            << "], mD[0x" << std::setw(2) << std::setfill('0') << std::hex << rhs.innerStruct.mD
            << "]";
        return os;
    }
};

int main()
{
    std::vector<std::string> words = {
        "Hello", "from", "GCC", __VERSION__, "!"
    };
    std::cout << words << std::endl;

    auto pFoo = std::make_unique<Foo>();
    std::cout << *pFoo << std::endl;
}