bslstl_stringviewlikeparam

Detailed Description

Provide macros for SFINAE constraints on string-view-like params.

Outline

• Purpose
• Macros
• Description
  • Macro Summary
  • Usage
    • Example 1: Declaring a String-View-Like Constructor

Purpose

Provide macros for SFINAE constraints on string-view-like params.

Macros

• BSLSTL_STRINGVIEWLIKEPARAM_TYPE_IF_COMPLETE: completeness check
• BSLSTL_STRINGVIEWLIKEPARAM_ENABLE_IF_T: SV-convertible enable_if
• BSLSTL_STRINGVIEWLIKEPARAM_ONLY_ENABLE_IF_T: SV-conv, not C-string

See also

bslstl_isconvertibletostringview, bslstl_isconvertibletocstring

Description

This component provides a set of macros that simplify the declaration of function template parameters constrained to accept "string-view-like" types. A type is considered "string-view-like" if it is convertible to bsl::basic_string_view<CHAR_TYPE, CHAR_TRAITS>.

The "ONLY" variants additionally exclude types that are convertible to const CHAR_TYPE * (C-strings), which is useful for overload resolution when separate overloads exist for C-string arguments.

These macros are designed to be used in function declarations and definitions where SFINAE is needed to constrain template parameters.

Macro Summary

This section provides a brief description of each macro:

• BSLSTL_STRINGVIEWLIKEPARAM_TYPE_IF_COMPLETE: Helper macro that provides a completeness check for the STRING_VIEW_LIKE_TYPE template parameter, working around SFINAE deficiencies in certain compilers.
• BSLSTL_STRINGVIEWLIKEPARAM_ENABLE_IF_T(ResultType): SFINAE enable_if type alias for types convertible to string_view. The result is ResultType if enabled. Use as: BSLSTL_STRINGVIEWLIKEPARAM_ENABLE_IF_T(void) * = 0 for declaration, or BSLSTL_STRINGVIEWLIKEPARAM_ENABLE_IF_T(const ALLOCATOR&) alloc = ALLOCATOR()
• BSLSTL_STRINGVIEWLIKEPARAM_ONLY_ENABLE_IF_T(ResultType): SFINAE enable_if type alias for types convertible to string_view but NOT convertible to C-string (const CHAR_TYPE *). The result is ResultType if enabled. Use as: BSLSTL_STRINGVIEWLIKEPARAM_ONLY_ENABLE_IF_T(void) * = 0 for declaration, or BSLSTL_STRINGVIEWLIKEPARAM_ONLY_ENABLE_IF_T(const ALLOCATOR&) alloc = ALLOCATOR()

Usage

This section illustrates intended use of this component.

Example 1: Declaring a String-View-Like Constructor

Suppose we want to declare a constructor that accepts any type convertible to bsl::string_view but not const char *.

First, we declare the constructor in the class definition:

template <class STRING_VIEW_LIKE_TYPE>
explicit MyClass(const STRING_VIEW_LIKE_TYPE& value,
                 BSLSTL_STRINGVIEWLIKEPARAM_ONLY_ENABLE_IF_T(void) *= 0);

Then, we define the constructor outside the class:

template <class STRING_VIEW_LIKE_TYPE>
MyClass::MyClass(const STRING_VIEW_LIKE_TYPE& value,
                 BSLSTL_STRINGVIEWLIKEPARAM_ONLY_ENABLE_IF_T(void) *)
{
    bsl::string_view sv = value;
    // use sv...
}

Macro Definition Documentation

BSLSTL_STRINGVIEWLIKEPARAM_ENABLE_IF_T

#define BSLSTL_STRINGVIEWLIKEPARAM_ENABLE_IF_T

(

...

)

Value:

    typename bsl::enable_if<                                                  \
        BloombergLP::bslstl::IsConvertibleToStringView<                       \
            CHAR_TYPE,                                                        \
            CHAR_TRAITS,                                                      \
            BSLSTL_STRINGVIEWLIKEPARAM_TYPE_IF_COMPLETE>::value,              \
        __VA_ARGS__>::type

Expand to an enable_if type alias for ResultType that is defined (i.e., SFINAE-friendly) only when the STRING_VIEW_LIKE_TYPE template parameter is convertible to bsl::basic_string_view<CHAR_TYPE, CHAR_TRAITS>. The CHAR_TYPE and CHAR_TRAITS template parameters must be in scope. The specified ResultType (passed as a variadic argument) is the resulting type when the condition is satisfied.

BSLSTL_STRINGVIEWLIKEPARAM_ONLY_ENABLE_IF_T

#define BSLSTL_STRINGVIEWLIKEPARAM_ONLY_ENABLE_IF_T

(

...

)

Value:

    typename bsl::enable_if<                                                  \
        BloombergLP::bslstl::IsConvertibleToStringView<                       \
            CHAR_TYPE,                                                        \
            CHAR_TRAITS,                                                      \
            BSLSTL_STRINGVIEWLIKEPARAM_TYPE_IF_COMPLETE>::value               \
     && !BloombergLP::bslstl::IsConvertibleToCString<                         \
            CHAR_TYPE,                                                        \
            BSLSTL_STRINGVIEWLIKEPARAM_TYPE_IF_COMPLETE>::value,              \
        __VA_ARGS__>::type

Expand to an enable_if type alias for ResultType that is defined (i.e., SFINAE-friendly) only when the STRING_VIEW_LIKE_TYPE template parameter is convertible to bsl::basic_string_view<CHAR_TYPE, CHAR_TRAITS> but is not convertible to const CHAR_TYPE *. The CHAR_TYPE and CHAR_TRAITS template parameters must be in scope. The specified ResultType (passed as a variadic argument) is the resulting type when both conditions are satisfied.

BSLSTL_STRINGVIEWLIKEPARAM_TYPE_IF_COMPLETE

#define BSLSTL_STRINGVIEWLIKEPARAM_TYPE_IF_COMPLETE

Value:

    typename bsl::enable_if<0 != sizeof(STRING_VIEW_LIKE_TYPE),               \
                            const STRING_VIEW_LIKE_TYPE&>::type

We need to use an intermediate completeness test to work around deficiencies with SFINAE in the Sun and AIX compilers.