Add 'Tweak header' support via "nonstd/span.tweak.hpp"

See: Tweak-header, A New Approach to Build-Time Library Configuration, by Colby Pike aka vector-of-bool. Oct 4, 2020
https://vector-of-bool.github.io/2020/10/04/lib-configuration.html

See also nonstd-lite/nonstd-lite#44

Author: martinmoene

README.md

@@ -192,6 +192,10 @@ To construct a span from a container with compilers that cannot constrain such a
 Configuration
 -------------
 
+### Tweak header
+
+If the compiler supports [`__has_include()`](https://en.cppreference.com/w/cpp/preprocessor/include), *span lite* supports the [tweak header](https://vector-of-bool.github.io/2020/10/04/lib-configuration.html) mechanism. Provide your *tweak header* as `nonstd/span.tweak.hpp` in a folder in the include-search-path. In the tweak header, provide definitions as documented below, like `#define span_CONFIG_NO_EXCEPTIONS 1`.
+
 ### Standard selection macro
 
 \-D<b>span\_CPLUSPLUS</b>=199711L  
@@ -433,6 +437,7 @@ span<>: Allows to construct from a const C-array with size via decay to pointer
 span<>: Allows to construct from a std::initializer_list<> (C++11)
 span<>: Allows to construct from a std::array<> (C++11)
 span<>: Allows to construct from a std::array<> with const data (C++11, span_FEATURE_CONSTR..._ELEMENT_TYPE=1)
+span<>: Allows to construct from an empty std::array<> (C++11)
 span<>: Allows to construct from a container (std::vector<>)
 span<>: Allows to tag-construct from a container (std::vector<>)
 span<>: Allows to tag-construct from a const container (std::vector<>)
@@ -505,4 +510,5 @@ tuple_size<>: Allows to obtain the number of elements via std::tuple_size<> (C++
 tuple_element<>: Allows to obtain an element via std::tuple_element<> (C++11)
 tuple_element<>: Allows to obtain an element via std::tuple_element_t<> (C++11)
 get<I>(spn): Allows to access an element via std::get<>()
+tweak header: reads tweak header if supported [tweak]
 ```

include/nonstd/span.hpp

@@ -26,6 +26,20 @@
 #define span_SPAN_NONSTD   1
 #define span_SPAN_STD      2
 
+// tweak header support:
+
+#ifdef __has_include
+# if __has_include(<nonstd/span.tweak.hpp>)
+#  include <nonstd/span.tweak.hpp>
+# endif
+#define span_HAVE_TWEAK_HEADER  1
+#else
+#define span_HAVE_TWEAK_HEADER  0
+//# pragma message("span.hpp: Note: Tweak header not supported.")
+#endif
+
+// span selection and configuration:
+
 #define span_HAVE( feature )  ( span_HAVE_##feature )
 
 #ifndef  span_CONFIG_SELECT_SPAN

test/CMakeLists.txt

@@ -15,6 +15,7 @@ set( unit_name "span" )
 set( PACKAGE   ${unit_name}-lite )
 set( PROGRAM   ${unit_name}-lite )
 set( SOURCES   ${unit_name}-main.t.cpp ${unit_name}.t.cpp )
+set( TWEAKD    "." )
 
 message( STATUS "Subproject '${PROJECT_NAME}', programs '${PROGRAM}-*'")
 
@@ -139,6 +140,7 @@ function( make_target target std )
     message( STATUS "Make target: '${std}'" )
 
     add_executable            ( ${target} ${SOURCES} ${HDRPATH} )
+    target_include_directories( ${target} PRIVATE ${TWEAKD} )
     target_link_libraries     ( ${target} PRIVATE ${PACKAGE} )
     target_compile_options    ( ${target} PRIVATE ${OPTIONS} )
     target_compile_definitions( ${target} PRIVATE ${DEFINITIONS} )

test/nonstd/span.tweak.hpp

@@ -0,0 +1 @@
+#define SPAN_TWEAK_VALUE 42

test/span.t.cpp

@@ -1551,7 +1551,7 @@ CASE( "tuple_element<>: Allows to obtain an element via std::tuple_element<> (C+
 #if span_HAVE( STRUCT_BINDING )
     using S = span<int,3>;
     using T = std::tuple_element<0, S>::type;
- 
+
     EXPECT( (std::is_same<T, int>::value) );
 
    static_assert( std::is_same<T, int>::value, "std::tuple_element<0, S>::type fails" );
@@ -1565,7 +1565,7 @@ CASE( "tuple_element<>: Allows to obtain an element via std::tuple_element_t<> (
 #if span_HAVE( STRUCT_BINDING ) && span_CPP11_140
     using S = span<int,3>;
     using T = std::tuple_element_t<0, S>;
- 
+
     EXPECT( (std::is_same<T, int>::value) );
 
    static_assert( std::is_same<T, int>::value, "std::tuple_element_t<0, S> fails" );
@@ -1606,9 +1606,9 @@ CASE( "get<I>(spn): Allows to access an element via std::get<>()" )
 
         // static_assert( std::get< 1 >( vc ) == 2, "std::tuple_element<I>(spn) fails" );
     }
-   
+
     SECTION("rvalue")
-    {    
+    {
         EXPECT( std::get< 1 >( std::move(vna) ) == 2 );
         EXPECT( std::get< 1 >( std::move(vnb) ) == 2 );
         EXPECT( std::get< 1 >( std::move(vca) ) == 2 );
@@ -1715,4 +1715,13 @@ CASE( "[hide][issue-3: same()]" )
 #endif // span_FEATURE_TO_STD( MAKE_SPAN )
 }
 
+CASE( "tweak header: reads tweak header if supported " "[tweak]" )
+{
+#if span_HAVE_TWEAK_HEADER
+    EXPECT( SPAN_TWEAK_VALUE == 42 );
+#else
+    EXPECT( !!"Tweak header is not available (span_HAVE_TWEAK_HEADER: 0)." );
+#endif
+}
+
 // end of file

test/t.bat

@@ -51,7 +51,7 @@ set CppCoreCheckInclude=^
 set byte_lite=^
     -Dspan_BYTE_LITE_HEADER=\"../../byte-lite/include/nonstd/byte.hpp\"
 
-cl -W3 -EHsc %std% %unit_select% %unit_contract% %unit_config% %msvc_defines% %byte_lite% -I"%CppCoreCheckInclude%" -I../include %unit%-main.t.cpp %unit%.t.cpp && %unit%-main.t.exe
+cl -W3 -EHsc %std% %unit_select% %unit_contract% %unit_config% %msvc_defines% %byte_lite% -I"%CppCoreCheckInclude%" -I../include -I. %unit%-main.t.cpp %unit%.t.cpp && %unit%-main.t.exe
 endlocal & goto :EOF
 
 :: subroutines:

test/tc.bat

@@ -50,7 +50,7 @@ rem -flto / -fwhole-program
 set  optflags=-O2
 set warnflags=-Wall -Wextra -Wpedantic -Weverything -Wno-c++98-compat -Wno-c++98-compat-pedantic -Wno-padded -Wno-missing-noreturn -Wno-documentation-unknown-command -Wno-documentation-deprecated-sync -Wno-documentation -Wno-weak-vtables -Wno-missing-prototypes -Wno-missing-variable-declarations -Wno-exit-time-destructors -Wno-global-constructors
 
-"%clang%" -m32 -std=%std% %optflags% %warnflags% %unit_select% %unit_contract% %unit_config% %byte_lite% -fms-compatibility-version=19.00 -isystem "%VCInstallDir%include" -isystem "%WindowsSdkDir_71A%include" -I../include -o %unit%-main.t.exe %unit%-main.t.cpp %unit%.t.cpp && %unit%-main.t.exe
+"%clang%" -m32 -std=%std% %optflags% %warnflags% %unit_select% %unit_contract% %unit_config% %byte_lite% -fms-compatibility-version=19.00 -isystem "%VCInstallDir%include" -isystem "%WindowsSdkDir_71A%include" -I../include -I. -o %unit%-main.t.exe %unit%-main.t.cpp %unit%.t.cpp && %unit%-main.t.exe
 endlocal & goto :EOF
 
 :: subroutines:

test/tg.bat

@@ -51,7 +51,7 @@ rem -flto / -fwhole-program
 set  optflags=-O2
 set warnflags=-Wall -Wextra -Wpedantic -Wconversion -Wsign-conversion -Wno-padded -Wno-missing-noreturn
 
-%gpp% -std=%std% %optflags% %warnflags% %unit_select% %unit_contract% %unit_config% %byte_lite% -o %unit%-main.t.exe -I../include %unit%-main.t.cpp %unit%.t.cpp && %unit%-main.t.exe
+%gpp% -std=%std% %optflags% %warnflags% %unit_select% %unit_contract% %unit_config% %byte_lite% -o %unit%-main.t.exe -I../include -I. %unit%-main.t.cpp %unit%.t.cpp && %unit%-main.t.exe
 
 endlocal & goto :EOF