声明boost::type_erasure::any
,即从字符串文字或char const*
构造和赋值会自动将字符串复制到std::string
中并将其存储在boost::type_erasure::any
对象中?
默认情况下,boost::type_erasure::any
只存储字符串指针。
目的是避免错误源,当我的any
类型的用户为其分配一个字符串指针时,假设将创建一个副本(就像std::string
一样(,然后字符串的生命周期在我的any
被读取之前结束,导致崩溃。
例:
#include <boost/type_erasure/operators.hpp>
#include <boost/type_erasure/any.hpp>
#include <boost/type_erasure/any_cast.hpp>
#include <boost/type_erasure/relaxed.hpp>
#include <boost/mpl/vector.hpp>
#include <iostream>
namespace te = boost::type_erasure;
using my_any = te::any< boost::mpl::vector<
te::copy_constructible<>,
te::destructible<>,
te::typeid_<>,
te::relaxed
/* I believe some changes here would do the trick */
>>;
using namespace std;
int main()
{
// Store an std::string by explicitly calling string constructor.
my_any a = string("abc");
// The following should copy-construct an std::string too but it just stores
// the string pointer.
my_any b = "abc";
// Works as expected.
cout << te::any_cast<string>( a ) << endl;
// This crashes because the underlying type of b is not std::string.
// With some changes to the my_any type this shouldn't crash anymore.
cout << te::any_cast<string>( b ) << endl;
}
现场演示。
不,any
存储任何内容。 const char*
什么都不是。
请注意,"hello"s
是类型 std::string
的文本。
我发布了我自己问题的答案,希望澄清boost::type_erasure::any
的预期用途,而不会使原始问题过于冗长。
此答案显示了通过将boost::type_erasure::any
隐藏在另一个类的接口后面并为可转换为 std::string
的所有类型的 setter 方法的重载来显示可能的解决方法。此重载负责将字符指针和字符数组转换为std::string
。
我认为一个额外的重载并不是那么糟糕,但理想情况下,我想避免使用样板文件并使any
类型知道如何转换 C 字符串。这让我们回到我最初的问题。
#include <iostream>
#include <unordered_map>
#include <string>
#include <type_traits>
#include <boost/type_erasure/operators.hpp>
#include <boost/type_erasure/any.hpp>
#include <boost/type_erasure/any_cast.hpp>
#include <boost/type_erasure/relaxed.hpp>
#include <boost/mpl/vector.hpp>
namespace te = boost::type_erasure;
// A class to store attributes of any type by name.
class Attributes
{
public:
using Key = std::string;
// A type that can store any value (similar to std::any).
using AnyValue = te::any< boost::mpl::vector<
te::copy_constructible<>,
te::destructible<>,
te::typeid_<>,
te::relaxed
>>;
// Overload for all types that ain't strings.
template< typename T >
std::enable_if_t< !std::is_convertible<T, std::string>::value,
void > SetAttr( Key const& name, T&& value )
{
m_attr.insert_or_assign( name, std::forward<T>( value ) );
}
// Convert to std::string for all convertible types
// (char pointer and char array included).
template< typename T >
std::enable_if_t< std::is_convertible<T, std::string>::value,
void > SetAttr( Key const& name, T&& value )
{
m_attr.insert_or_assign( name, std::string( std::forward<T>( value ) ) );
}
template< typename T >
T GetAttr( Key const& name ) const
{
return te::any_cast<T>( m_attr.at( name ) );
}
private:
std::unordered_map<Key, AnyValue> m_attr;
};
以下示例显示了如何将不同类型的字符串传递给Attributes::SetAttr()
并通过 Attributes::GetAttr<std::string>()
进行一般查询:
using namespace std;
Attributes a;
// Works even w/o special care.
a.SetAttr( "key1", string("foo") );
// Without the SetAttr() overload for strings, user would have to remind
// to cast back to char const* when calling MyClass::GetAttr().
a.SetAttr( "key2", "bar" );
// Without the SetAttr() overload for strings, a later call to GetAttr()
// would cause a crash because we are passing pointers to temporary objects.
{
// test arrays
char temp1[] = { 'b', 'a', 'z', 0 };
a.SetAttr( "key3", temp1 );
char const temp2[] = { 'b', 'i', 'm', 0 };
a.SetAttr( "key4", temp2 );
// test pointers
a.SetAttr( "key5", &temp1[0] );
a.SetAttr( "key6", &temp2[0] );
}
try
{
// When getting a string attribute we no longer have to care about how it was
// passed to SetAttr(), we can simply cast to std::string in all cases.
cout << "'" << a.GetAttr<string>( "key1" ) << "'" << endl;
cout << "'" << a.GetAttr<string>( "key2" ) << "'" << endl;
cout << "'" << a.GetAttr<string>( "key3" ) << "'" << endl;
cout << "'" << a.GetAttr<string>( "key4" ) << "'" << endl;
cout << "'" << a.GetAttr<string>( "key5" ) << "'" << endl;
cout << "'" << a.GetAttr<string>( "key6" ) << "'" << endl;
}
// boost::type_erasure::bad_any_cast or std::out_of_range
catch( std::exception& e )
{
cout << "Error: " << e.what() << endl;
}
现场演示。