NAME¶

deductionguidesforstd::basic_string - deductionguidesforstd::basic_string

Synopsis¶

Defined in header <string>
template< class InputIt, class Alloc = std::allocator<

typename std::iterator_traits<InputIt>::value_type> >
basic_string( InputIt, InputIt, Alloc = Alloc() )
-> basic_string<typename (1) (since C++17)
std::iterator_traits<InputIt>::value_type,
std::char_traits<typename
std::iterator_traits<InputIt>::value_type>,

Alloc>;
template< class CharT,

class Traits,
class Alloc = std::allocator<CharT> > (2) (since C++17)
explicit basic_string( std::basic_string_view<CharT, Traits>,
const Alloc& = Alloc() )

-> basic_string<CharT, Traits, Alloc>;
template< class CharT,

class Traits,
class Alloc = std::allocator<CharT>> >
basic_string( std::basic_string_view<CharT, Traits>, typename (3) (since C++17)
/*see below*/::size_type,
typename /*see below*/::size_type, const Alloc& = Alloc() )

-> basic_string<CharT, Traits, Alloc>;

1) This deduction guide is provided for std::basic_string to allow deduction from an
iterator range. This overload participates in overload resolution only 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
participate in overload resolution only 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_views are made templates to avoid causing ambiguities in existing
code, and those templates do not support class template argument deduction.

Example¶

// Run this code

#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
}

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 3075 C++17 deduction from basic_string_view was deduction guides
unsupported (exacerbated by LWG issue 2946) added