C++内部变量的奇怪模板特化

Question

考虑以下代码:

#include    <iostream>
#include    <typeinfo>
#include    <cstring>
#include    <cstdlib>

static void random_string( char *s, const int len )
{
    static const char alphanum[] =
        "0123456789"
        "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
        "abcdefghijklmnopqrstuvwxyz";

    for ( int i = 0; i < len; ++i )
        s[ i ] = alphanum[ rand( ) % ( sizeof( alphanum ) - 1 ) ];

    s[ len ] = 0;
}

template< typename Type, int Length >
void    info( Type ( &var )[ Length ] )
{
    std::cout << "->    Type is " << typeid( var ).name( ) << std::endl;
    std::cout << "->    Pointer size:   " << sizeof( Type * ) << std::endl;
    std::cout << "->    Number of elements: " << Length << std::endl;
    std::cout << "->    Size of each element:   " << sizeof( Type ) << std::endl;
    std::cout << "->    contents: \"" << var << "\"." << std::endl;
}


template< typename SourceType, int SourceLength, typename TargetType, int TargetLength >
void    func( TargetType ( &target )[ TargetLength ] )
{
    SourceType  source[ SourceLength ];

    random_string( ( char * )source, SourceLength - 1 );

    std::cout << "-= source =-" << std::endl;
    info( source );

    std::cout << "-= target =-" << std::endl;
    info( target );

    std::memcpy( target, source,
            ( SourceLength < TargetLength ? SourceLength : TargetLength ) );
}

int main( )
{
    typedef char    char16[ 16 ];
    typedef char    char64[ 64 ];

    char16  c16 = "16 bytes chars.";
    char64  c64 = "64 bytes chars. There's a lot more room here...";

    std::cout << "c16 = \"" << c16 << "\"." << std::endl;
    func< char64, sizeof( char64 ) >( c16 );
    std::cout << "c16 = \"" << c16 << "\"." << std::endl;

    std::cout << "c64 = \"" << c64 << "\"." << std::endl;
    func< char16, sizeof( char16 ) >( c64 );
    std::cout << "c64 = \"" << c64 << "\"." << std::endl;

    return  0;
}

这个输出:

> g++ -Wall -g3 array_conversions.cpp -o array_conversions
> ./array_inner_type
c16 = "16 bytes chars.".
-= source =-
->      Type is A64_A64_c
->      Pointer size:   8
->      Number of elements:     64
->      Size of each element:   64
->      contents: "0x7fff8b913380".
-= target =-
->      Type is A16_c
->      Pointer size:   8
->      Number of elements:     16
->      Size of each element:   1
->      contents: "16 bytes chars.".
c16 = "fa37JncCHryDsbza".
c64 = "64 bytes chars. There's a lot more room here...".
-= source =-
->      Type is A16_A16_c
->      Pointer size:   8
->      Number of elements:     16
->      Size of each element:   16
->      contents: "0x7fff8b914280".
-= target =-
->      Type is A64_c
->      Pointer size:   8
->      Number of elements:     64
->      Size of each element:   1
->      contents: "64 bytes chars. There's a lot more room here...".
c64 = "Z2nOXpPIhMFSv8k".

可以看出，我和GCC对SourceType source[ SourceLength ]的理解是不一样的。

我希望 SourceType source[ SourceLength ] 是 char64 ("A64_c") 类型，但 GCC 告诉我它是 "A64_A64_c"并且表现得好像它是char64 或类似内容的数组。

奇怪的是，它只发生在本地 source 变量上。 target 函数参数被正确解释。

我错过了什么？我的代码有什么误解吗？

我觉得这个问题与这个 C++ Template argument changes Reference to Pointer 有关，我大约一年前发布的，但是这个问题无论如何都没有答案，所以现在我请求你的帮助。

非常感谢。

Answer 1

c16类型为 char[16]

func< char64, sizeof( char64 ) >( c16 )实例化 func与

SourceType == char64 == char[64]
SourceLength == sizeof( char64 ) == 64
TargetType == char
TargetLength == 16

这意味着target类型为 char (&)[16]和 source类型为 char[64][64] .

这解释了为什么 target是 char 的数组, 和 source是 char 的数组数组