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std::get_if

STD：得到[医]如果

Defined in header ​​
​(1)(since C++17)
template <std::size_t I, class... Types> constexpr std::add_pointer_t< std::variant_alternative_t<I, std::variant<Types...>> > get_if(std::variant<Types...>* pv)​
template <std::size_t I, class... Types> constexpr std::add_pointer_t< const std::variant_alternative_t<I, variant<Types...>> > get_if(const std::variant<Types...>* pv)​
​(2)(since C++17)
template <class T, class... Types> constexpr std::add_pointer_t<T> get_if(variant<Types...>* pv)​
template <class T, class... Types> constexpr std::add_pointer_t<const T> get_if(const variant<Types...>* pv)​

1%29基于索引的非抛出访问器：如果pv不是空指针，并且pv->index() == I的变量中存储的值的指针。pv否则，返回空指针值。如果调用不正确，则为I变量中的有效索引，或者T_I是%28可能是cv-合格%29类型void...

基于2%29类型的非抛出访问器：等效于%281%29I的零基索引。T在Types......打电话是不正确的，如果T的唯一元素Types...或者如果T_I是%28可能是cv-合格%29类型void...

参数

I-index to look up
Type-unique type to look up
pv-pointer to a variant

返回值

指向错误时存储在指向变量或空指针中的值的指针。

例外

1，2%29

noexcept规格：

noexcept

例

二次

#include <variant>
#include <iostream>
 
int main()
{
    std::variant<int, float> v{12};
 
    if(auto pval = std::get_if<int>(&v))
      std::cout << "variant value: " << *pval << '\n'; 
    else 
      std::cout << "failed to get value!" << '\n'; 
}

二次

产出：

二次

variant value: 12

二次

另见

std::get(std::variant) (C++17)reads the value of the variant given the index or the type (if the type is unique), throws on error (function template)

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