deduction guides for std::basic_string
deduction guides for std::basic_string
Defined in header <string>
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(1) | (since C++17) |
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(2) | (since C++17) |
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(3) | (since C++17) |
1) This deduction guide is provided for std::basic_string
to allow deduction from an iterator range. This overload only participates in overload resolution if InputIt
satisfies LegacyInputIterator and Alloc
satisfies Allocator.
2-3) These deduction guides are provided for std::basic_string
to allow deduction from a std::basic_string_view
. The size_type
parameter type in (3) refers to the size_type
member type of the type deduced by the deduction guide. These overloads only participate in overload resolution if Alloc
satisfies Allocator.
Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type Alloc::value_type
must exist and the expression std::declval<Alloc&>().allocate(std::size_t{})
must be well-formed when treated as an unevaluated operand.
Notes
Guides (2-3) are needed because the std::basic_string
constructors for std::basic_string_view
s are made templates to avoid causing ambiguities in existing code, and those templates do not support class template argument deduction.
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
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LWG 3075 | C++17 | deduction from basic_string_view was unsupported (exacerbated by LWG issue 2946)
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deduction guides added |
Example
#include <string>
#include <vector>
int main() {
std::vector<char> v = {'a', 'b', 'c'};
std::basic_string s(v.begin(), v.end()); // uses explicit deduction guide
}
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