C++ 中map集合更新value报错 Assigning to already instantiated xmlrpc_c::value

Question

代码中定义了一个map：

std::map<int64, std::vector<xmlrpc_c::value> > entrust;

key是int 类型，value是vector类型，vector中保存xmlrpc_c::value类型的值，xmlrpc_c是xmlrpc框架中的一个数据类型，xmlrpc_c::value类型在xmlrcp框架中是这样定义的

namespace xmlrpc_c {

class XMLRPC_LIBPP_EXPORTED value {
    // This is a handle.  You don't want to create a pointer to this;
    // it is in fact a pointer itself.
public:
    value();
        // This creates a placeholder.  It can't be used for anything, but
        // holds memory.  instantiate() can turn it into a real object.

    value(xmlrpc_c::value const &value);  // copy constructor

    ~value();

    enum type_t {
        // These are designed to be identical to the values for
        // enum xmlrpc_type in the C library.
        TYPE_INT        = 0,
        TYPE_BOOLEAN    = 1,
        TYPE_DOUBLE     = 2,
        TYPE_DATETIME   = 3,
        TYPE_STRING     = 4,
        TYPE_BYTESTRING = 5,
        TYPE_ARRAY      = 6,
        TYPE_STRUCT     = 7,
        TYPE_C_PTR      = 8,
        TYPE_NIL        = 9,
        TYPE_I8         = 10,
        TYPE_DEAD       = 0xDEAD
    };

    type_t type() const;

    xmlrpc_c::value&
    operator=(xmlrpc_c::value const&);

    bool
    isInstantiated() const;

    // The following are not meant to be public to users, but just to
    // other Xmlrpc-c library modules.  If we ever go to a pure C++
    // implementation, not based on C xmlrpc_value objects, this shouldn't
    // be necessary.

    void
    appendToCArray(xmlrpc_value * const arrayP) const;

    void
    addToCStruct(xmlrpc_value * const structP,
                 std::string    const key) const;

    xmlrpc_value *
    cValue() const;
        // Not to be confused with public 'cvalue' method that all the derived
        // classes have.

    value(xmlrpc_value * const valueP);

    void
    instantiate(xmlrpc_value * const valueP);
        // Works only on a placeholder object created by the no-argument
        // constructor.

    xmlrpc_value * cValueP;
        // NULL means this is merely a placeholder object.

protected:
    void
    validateInstantiated() const;
};

typedef std::vector<xmlrpc_c::value> carray;

class XMLRPC_LIBPP_EXPORTED value_array : public value {
public:
    value_array(carray const& cvalue);

    value_array(xmlrpc_c::value const baseValue);

    // You can't cast to a vector because the compiler can't tell which
    // constructor to use (complains about ambiguity).  So we have this:
    carray
    vectorValueValue() const;

    carray cvalue() const;

    size_t
    size() const;
};

在我写的代码中使用 it->second = value;方式更新value值，编译没有报错，执行时就会报错

std::vector<xmlrpc_c::value> value;
 value.push_back(xmlrpc_c::value_int(order_no));
 value.push_back(xmlrpc_c::value_int(hh));
 value.push_back(xmlrpc_c::value_int(mm));
std::map<int64, int>::iterator it_cancel = cancel_entrust.find(order_no);
    if (it_cancel == cancel_entrust.end())
        cancel_entrust.insert(std::make_pair(order_no, stock_code));
    else
        it_cancel->second = stock_code;
   // entrust_mtx.lock();
    std::map<int64, std::vector<xmlrpc_c::value> >::iterator it = entrust.find(order_no);
    //map中不存在
    if (it == entrust.end())
    {
        entrust.insert(std::make_pair(order_no, value));
    }
    else
    {
       it->second = value;
    }

执行时，更新map中value值就会报错 报错信息为 Assigning to already instantiated xmlrpc_c::value

这个不知道是什么问题，大概猜测是没有重写操作符，但是重写操作符只与key值有关，也不知道如何去重写操作符，
求各位大 牛看一下，为什么更新map中的value值就会报错