参考stackoverflow: converting _int64 to a string，验证建议的代码可以成功地将_int64转换到string。

C++ 11的标准做法：

#include <string>

int main()
{
  int64_t value = 100;
  std::string asString = std::to_string(value);
  return 0;
}

使用stringstream:

#include <string>
#include <sstream>

int main()
{
  std::stringstream stream;
  __int64 value(1000000000);
  stream << value;    
  std::string strValue(stream.str());
  return 0;
}

使用C sprintf() 或者 _i64toa()

__int64 value = ...;
char buffer[20];
sprintf(buffer, "%Ld", value);

__int64 value = ...;
char buffer[20];
_i64toa(value, buffer, 10);

参考

stackoverflow: converting _int64 to a string
Microsoft Docs: _itoa, _i64toa, _ui64toa, _itow, _i64tow, _ui64tow