c++ - 为什么在这里使用 int64_t 错误并编译 int？

Question

帮帮小弟。这段代码编译，做我想做的事。

#include <iostream>
#include <unordered_map>
#include <variant>
#include <vector>
#include <random>
#include <ctime>

class GlobalVariables
{
public:
    typedef std::wstring VariableNameType;


    /***************************************************************
    ----------------------------------------------------------------------

    Important that my std::variant holds an int

    -----------------------------------------------------------------
    *****************************************************************/


    typedef std::variant<bool,int,float,std::wstring> VariableType;
    typedef std::unordered_map<VariableNameType,VariableType> VariableContainerType;

    static const VariableType& GetValue(const VariableNameType& name) { return m_variables[name]; }
    static void SetValue(const VariableNameType& name, const VariableType value) { m_variables[name] = value; }

protected:
    static VariableContainerType m_variables;
};

GlobalVariables::VariableContainerType GlobalVariables::m_variables;

class RandomNumber
{
public:
    typedef std::mt19937 EngineType;

    static void Seed(const EngineType::result_type seed)
    {
        m_engine.seed(seed);
    }

protected:
    static EngineType m_engine;
};

RandomNumber::EngineType RandomNumber::m_engine(std::time(0));

template<typename T>
class RandomNumberGenerator : public RandomNumber
{ };

template<>
class RandomNumberGenerator<int> : public RandomNumber
{
public:
    RandomNumberGenerator(const int min, const int max)
    :
        m_intDistro(min,max)
    {}

    int operator()() { return m_intDistro(m_engine); }

protected:

    std::uniform_int_distribution<int> m_intDistro;
};

template<>
class RandomNumberGenerator<float> : public RandomNumber
{
public:
    RandomNumberGenerator(const float min, const float max)
    :
        m_floatDistro(min,max)
    {}

    float operator()() { return m_floatDistro(m_engine); }

protected:

    std::uniform_real_distribution<float> m_floatDistro;
};


enum class COMPARISON_OP : uint8_t { EE, NE, LT, GT, LE, GE };
enum class BINARY_OP : uint8_t { ADD, SUB, MUL, DIV };

class Expression
{
public:
    virtual GlobalVariables::VariableType GetValue() const = 0;
};

class Constant : public Expression
{
public:
    virtual GlobalVariables::VariableType GetValue() const
    {
        return value;
    }

    GlobalVariables::VariableType value;
};

class Identifier : public Expression
{
public:
    virtual GlobalVariables::VariableType GetValue() const override
    {
        return GlobalVariables::GetValue(name);
    }

    GlobalVariables::VariableNameType name;
};

class System : public Expression
{
public:
    virtual GlobalVariables::VariableType GetValue() const override
    {
        return GlobalVariables::GetValue(name);
    }

    GlobalVariables::VariableNameType name;
};



class BinaryOp : public Expression
{
public:
    virtual GlobalVariables::VariableType GetValue() const override
    {
        if(op == BINARY_OP::ADD)



    /***************************************************************
    ----------------------------------------------------------------------

    Important that my visitor returns a simple addition if the types are the 
    same

    -----------------------------------------------------------------
    *****************************************************************/



            return std::visit([](auto a, auto b) -> GlobalVariables::VariableType
            {
                if constexpr(!std::is_same_v<decltype(a),decltype(b)>)
                    throw;
                else
                    return a + b;
            }, lhs->GetValue(), rhs->GetValue());
        else if(op == BINARY_OP::SUB)
            return 0;
        else if(op == BINARY_OP::MUL)
            return 0;
        else // Division
            return 0;
    }

    BINARY_OP op;
    Expression* lhs;
    Expression* rhs;
};

class Comparison
{
public:
    bool GetValue() const
    {
        if(op == COMPARISON_OP::EE)
            return lhs->GetValue() == rhs->GetValue();
        else if(op == COMPARISON_OP::NE)
            return lhs->GetValue() != rhs->GetValue();
        else if(op == COMPARISON_OP::LT)
            return lhs->GetValue() < rhs->GetValue();
        else if(op == COMPARISON_OP::GT)
            return lhs->GetValue() > rhs->GetValue();
        else if(op == COMPARISON_OP::LE)
            return lhs->GetValue() <= rhs->GetValue();
        else // Greater or Equal to
            return lhs->GetValue() >= rhs->GetValue();
    }

    COMPARISON_OP op;
    Expression* lhs;
    Expression* rhs;
};

class Random : public Expression
{
public:
    virtual GlobalVariables::VariableType GetValue() const override
    {
        return possibilities[randomSelector()]->GetValue();
    }

    mutable RandomNumberGenerator<int> randomSelector;

    std::vector<Expression*> possibilities;
};

class Option
{

};

class Event
{
public:
    Expression* eventText;
};

class Branch
{
public:
    const std::wstring& GetValue() const
    {
        if(condition->GetValue())
            return trueBody;
        else
            return falseBody;
    }

    Comparison* condition;
    std::wstring trueBody;
    std::wstring falseBody;
};

int main()
{

}

可能有很多不必要的代码，所以我突出显示了我认为重要的部分。

但是，如果我改变我的 std::variant持有int64_t而不是 int (不是“除了”，而是“代替”)即 typedef std::variant<bool,int64_t,float,std::wstring> VariableType程序编译失败。

编译错误很长:

||=== Build: Debug in Space Adventure 2 (compiler: Mingw-7.3.0) ===|
main.cpp||In instantiation of 'BinaryOp::GetValue() const::<lambda(auto:1, auto:2)> [with auto:1 = bool; auto:2 = bool; GlobalVariables::VariableType = std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >]':|
\include\c++\type_traits|2797|required from 'constexpr bool std::__call_is_nt(std::__invoke_other) [with _Fn = BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>; _Args = {bool, bool}]'|
\include\c++\type_traits|2803|  required by substitution of 'template<bool __v> using __bool_constant = std::integral_constant<bool, __v> [with bool __v = std::__call_is_nt<BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>, bool, bool>((std::__result_of_success<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, std::__invoke_other>::__invoke_type{}, std::__result_of_success<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, st|
\include\c++\type_traits|2801|required from 'struct std::__call_is_nothrow<std::__invoke_result<BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>, bool, bool>, BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>, bool, bool>'|
\include\c++\type_traits|143|required from 'struct std::__and_<std::__is_invocable<BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>, bool, bool>, std::__call_is_nothrow<std::__invoke_result<BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>, bool, bool>, BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>, bool, bool> >'|
\include\c++\type_traits|2813|required from 'struct std::__is_nothrow_invocable<BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>, bool, bool>'|
\include\c++\bits\invoke.h|89|  [ skipping 2 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]|
\include\c++\variant|691|required from 'static constexpr auto std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<_Result_type (*)(_Visitor, _Variants ...)>, std::tuple<_Tail ...>, std::integer_sequence<long long unsigned int, __indices ...> >::_S_apply() [with _Result_type = std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >; _Visitor = BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>&&; _Variants = {std::variant<bo|
\include\c++\variant|656|  recursively required from 'static constexpr void std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<_Result_type (*)(_Visitor, _Variants ...), __dimensions ...>, std::tuple<_Variants ...>, std::integer_sequence<long long unsigned int, __indices ...> >::_S_apply_single_alt(_Tp&) [with long long unsigned int __index = 0; _Tp = std::__detail::__variant::_Multi_array<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::al|
\include\c++\variant|656|required from 'constexpr const std::__detail::__variant::_Multi_array<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > > (*)(BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::alloc|
\include\c++\variant|709|required from 'struct std::__detail::__variant::__gen_vtable<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t|
\include\c++\variant|1245|required from 'constexpr decltype(auto) std::visit(_Visitor&&, _Variants&& ...) [with _Visitor = BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>; _Variants = {std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >}]'|
main.cpp|133|required from here|
\include\c++\variant|709|  in constexpr expansion of 'std::__detail::__variant::__gen_vtable<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >, BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<|
\include\c++\variant|706|  in constexpr expansion of 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > > (*)(BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wcha|
\include\c++\variant|646|  in constexpr expansion of 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > > (*)(BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wcha|
\include\c++\variant|668|  in constexpr expansion of 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > > (*)(BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wcha|
\include\c++\variant|646|  in constexpr expansion of 'std::__detail::__variant::__gen_vtable_impl<std::__detail::__variant::_Multi_array<std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > > (*)(BinaryOp::GetValue() const::<lambda(auto:1, auto:2)>&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >&&, std::variant<bool, long long int, float, std::__cxx11::basic_string<wcha|
main.cpp|132|error: could not convert '(((int)a) + ((int)b))' from 'int' to 'GlobalVariables::VariableType {aka std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >}'|
main.cpp||In member function 'virtual GlobalVariables::VariableType BinaryOp::GetValue() const':|
main.cpp|135|error: could not convert '0' from 'int' to 'GlobalVariables::VariableType {aka std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >}'|
main.cpp|137|error: could not convert '0' from 'int' to 'GlobalVariables::VariableType {aka std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >}'|
main.cpp|139|error: could not convert '0' from 'int' to 'GlobalVariables::VariableType {aka std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >}'|
||=== Build failed: 12 error(s), 10 warning(s) (0 minute(s), 1 second(s)) ===|

但我认为真正的关键是这个错误:

\main.cpp|132|error: could not convert '(((int)a) + ((int)b))' from 'int' to 'GlobalVariables::VariableType {aka std::variant<bool, long long int, float, std::__cxx11::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> > >}'|

我认为这很重要，因为我在任何时候都没有使用 int ，我正在使用 int64_t所以看起来程序已经从 int64_t 隐式转换了下降到 int我看不出在哪里、为什么或如何阻止它。

使用 Mingw64 7.3.0 编译。

Answer 1

问题出在 BinaryOp::GetValue 函数中的 return 语句。第一个错误来自 return a + b; 行，当展开变量的 bool 成员时。两个 bool 值被转换为 int，它们被加在一起，然后 int 被用来创建一个新的 GlobalVariables::VariableType 值。但是，没有从 int 到 VariableType 的明确转换。

出于同样的原因，其他错误都来自各种 return 0; 语句。

修复非常简单。把第一个return改成

return decltype(a)(a + b);

这会将加法的结果转换为与 a 相同的类型，这对于在加法发生之前提升为 int 的小类型是必需的(bool、char、short 及其有符号/无符号变体)。

对于 return 0; 行，您需要确定 0 应该是什么类型(bool？int_64？float？)并明确指定该 0 的类型.