Question

#include <boost/any.hpp>
#include <list>
#include <string>
#include <vector>

struct _time_t {
    int month;
    int year;
};


int main()
{
    std::string str = "hahastr";
    _time_t t;
    std::vector<boost::any> objVec;
    objVec.push_back(1);
    char* pstr = "haha";
    //boost::any charArr = "haha"; not compile
    //objVec.push_back("haha"); not compile
    objVec.push_back(pstr);
    objVec.push_back(str);
    objVec.push_back(t);
    return 0;
};

注释的代码行不编译，为什么？我认为在大多数情况下字符串文字可以充当char *，这是相关的r值和l-rvalue吗？

错误讯息： test_boost_any.cc

D:\Program Files (x86)\Microsoft Visual Studio 11.0\VC\INCLUDE\xlocale(336) : wa
rning C4530: C++ exception handler used, but unwind semantics are not enabled. S
pecify /EHsc
e:\projects\framework\boost_1_53_0\boost/any.hpp(122) : error C2536: 'boost::any
::holder<ValueType>::boost::any::holder<ValueType>::held' : cannot specify expli
cit initializer for arrays
        with
        [
            ValueType=const char [5]
        ]
        e:\projects\framework\boost_1_53_0\boost/any.hpp(139) : see declaration
of 'boost::any::holder<ValueType>::held'
        with
        [
            ValueType=const char [5]
        ]
        e:\projects\framework\boost_1_53_0\boost/any.hpp(120) : while compiling
class template member function 'boost::any::holder<ValueType>::holder(ValueType
(&))'
        with
        [
            ValueType=const char [5]
        ]
        e:\projects\framework\boost_1_53_0\boost/any.hpp(47) : see reference to
function template instantiation 'boost::any::holder<ValueType>::holder(ValueType
 (&))' being compiled
        with
        [
            ValueType=const char [5]
        ]
        e:\projects\framework\boost_1_53_0\boost/any.hpp(46) : see reference to
class template instantiation 'boost::any::holder<ValueType>' being compiled
        with
        [
            ValueType=const char [5]
        ]
        test_boost_any.cc(19) : see reference to function template instantiation
 'boost::any::any<const char[5]>(ValueType (&))' being compiled
        with
        [
            ValueType=const char [5]
        ]

Answer 1

string-literal不是指针，在你的情况下是array of N const char，因为boost::any构造函数接收T（推导到char[5]，而不是const char* 1}}，数组到指针的转换在这里不起作用），但你不能用initializer-list中的另一个数组初始化数组。

Answer 2

Boost.any值必须有效才能分配（ValueType的要求）。但是，字符串文字是一个数组，并且不能在C ++中分配数组。

如果需要，您可以将文字投射到const char *。

Answer 3

C中 crufty数组语义的最简单的解决方法是

boost::any charArr = +"haha";

注意使用+隐式将char数组衰减为const char*

其他人用数组值语义解释了这个问题

Answer 4

编译器告诉你它不能接受数组，例如VS2010会告诉你：

1>D:\SRC\CDR\Trunk\DRIT\ThirdParty\boost/any.hpp(122): error C2536: 'boost::any::holder<ValueType>::boost::any::holder<ValueType>::held' : cannot specify explicit initializer for arrays
1>          with
1>          [
1>              ValueType=const char [5]
1>          ]
1>          D:\SRC\CDR\Trunk\DRIT\ThirdParty\boost/any.hpp(139) : see declaration of 'boost::any::holder<ValueType>::held'
1>          with
1>          [
1>              ValueType=const char [5]
1>          ]
1>          D:\SRC\CDR\Trunk\DRIT\ThirdParty\boost/any.hpp(120) : while compiling class template member function 'boost::any::holder<ValueType>::holder(ValueType (&))'
1>          with
1>          [
1>              ValueType=const char [5]
1>          ]
1>          D:\SRC\CDR\Trunk\DRIT\ThirdParty\boost/any.hpp(46) : see reference to class template instantiation 'boost::any::holder<ValueType>' being compiled
1>          with
1>          [
1>              ValueType=const char [5]
1>          ]
1>          toto.cpp(20) : see reference to function template instantiation 'boost::any::any<const char[5]>(ValueType (&))' being compiled
1>          with
1>          [
1>              ValueType=const char [5]
1>          ]

“哈哈”的类型不是const char*而是const char[5]。如果将字符串转换为char*，则会编译：

boost::any charArr = static_cast<const char*>("haha"); // will compile

或者，您可以只存储std::string。我怀疑这是因为数组不能存储为指针。您也可以使用boost::array或std::array（如果有的话）。

Here是在Boost中添加数组支持的讨论的链接。

Answer 5

此问题的简化版本：

template <typename T>
class Array
{
    public:
        Array(T value) : value_(value) {}  
    private:
        T value_;
};

int main()
{
    int a[5] = {1,2,3,4,5};
    Array<int[5]> arr = a;
    return 0;
}

为什么boost :: any不包含字符串文字？

5 个答案: