[英]how to overload empty std::initializer_list?
Here I have series of overloaded functions that have as input either vector or initializer_list. 这里我有一系列重载函数,它们都是vector或initializer_list的输入。 And I want to handle the special case when client code inputs empty initializer_list.
我想在客户端代码输入空的initializer_list时处理特殊情况。 The problem is that compiler cannot determine what data was supposed in such empty list.
问题是编译器无法确定这种空列表中的数据。 So my question is how I address in function declaration such case.
所以我的问题是我如何在函数声明中解决这种情况。
#include <string>
#include <vector>
using namespace std;
void func(vector<string> v) { }
void func(vector<wstring> v) { }
void func(initializer_list<string> iv) {}
void func(initializer_list<wstring> iv) {}
int main() {
using namespace std;
func({"apple", "banana"});
func({L"蘋果", L"香蕉"});
func({}); // special case
}
error message: 错误信息:
<stdin>: In function 'int main()':
<stdin>:14:10: error: call of overloaded 'func(<brace-enclosed initializer list>)' is ambiguous
<stdin>:14:10: note: candidates are:
<stdin>:5:6: note: void func(std::vector<std::basic_string<char> >)
<stdin>:6:6: note: void func(std::vector<std::basic_string<wchar_t> >)
<stdin>:7:6: note: void func(std::initializer_list<std::basic_string<char> >)
<stdin>:8:6: note: void func(std::initializer_list<std::basic_string<wchar_t> >)
void func(initializer_list<void> iv) {}
- has no effect. void func(initializer_list<void> iv) {}
- 无效。 I don't know how to properly declare it. 我不知道如何正确宣布它。
I'm hoping there's a better answer, but you can exploit the fact that list-initialization with empty braces of a defined type prefers a default constructor to an initializer_list
constructor. 我希望有一个更好的答案,但你可以利用这样一个事实,即使用已定义类型的空括号的列表初始化更喜欢
initializer_list
构造函数的initializer_list
构造函数。 Unfortunately, this means having to turn the overload of func
into an overload set of constructors of a single parameter type: 不幸的是,这意味着必须将
func
的重载转换为单个参数类型的重载构造函数:
using namespace std;
struct parameter {
// representation left as an exercise - perhaps Boost.Variant?
parameter(vector<string> v) {}
parameter(vector<wstring> v) {}
parameter(initializer_list<string> iv) {}
parameter(initializer_list<wstring> iv) {}
parameter() {} // default constructor - func({}) calls this
};
void func(parameter) {}
If you weren't using string
, you could use a template ie template<typename T> void func(initializer_list<T>)
with an empty
overload, as non-template overloads are preferred to template overloads, but unfortunately in your case the string literal calls are inferred as initializer_list<const char *>
, which is difficult to convert to initializer_list<string>
. 如果您没有使用
string
,则可以使用模板,即template<typename T> void func(initializer_list<T>)
其中包含empty
重载,因为非模板重载优先于模板重载,但不幸的是在您的情况下是字符串文字调用被推断为initializer_list<const char *>
,很难转换为initializer_list<string>
。
There's no way to distinguish this with the parameter alone. 没有办法单独用参数来区分它。 You can make one of them a template though, which will then be more costly and less preferable for overload resolution
您可以将其中一个作为模板,然后这将成本更高,并且不太适合重载解析
void func(vector<string> v) { }
void func(vector<wstring> v) { }
template<typename = void>
void func(initializer_list<string> iv) {}
void func(initializer_list<wstring> iv) {}
Now calling func({})
will prefer the last function over the function template. 现在调用
func({})
将优先于函数模板的最后一个函数。 Note that func({"hello", "world"})
still prefers the function template over the non-template vector-taking function, because the parameter conversion cost is more important than whether or not a candidate was synthesized from a template. 请注意,
func({"hello", "world"})
仍然比非模板向量捕获函数更喜欢函数模板,因为参数转换成本比是否从模板合成候选更重要。
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