[build] Fixed possibility to compile examples with C++03 standard (#1830

)

CMakeLists.txt

@@ -415,9 +415,6 @@ if (NOT ENABLE_CXX11)
 	if (ENABLE_STDCXX_SYNC)
 		message(FATAL_ERROR "ENABLE_STDCXX_SYNC is set, but C++11 is disabled by ENABLE_CXX11")
 	endif()
-	if (DEFINED USE_CXX_STD)
-		message(FATAL_ERROR "USE_CXX_STD can be set only when ENABLE_CXX11 is on")
-	endif()
 elseif (ENABLE_STDCXX_SYNC)
 	add_definitions(-DENABLE_STDCXX_SYNC=1)
 	if (DEFINED USE_CXX_STD)
@@ -461,9 +458,14 @@ if (DEFINED USE_CXX_STD)
 
 			# Set this through independent flags
 			set (USE_CXX_STD_LIB ${STDCXX})
-			set (USE_CXX_STD_APP "")
 			set (FORCE_CXX_STANDARD 1)
-			message(STATUS "C++ STANDARD: library: C++${STDCXX}, but apps still at least C++11")
+			if (NOT ENABLE_APPS)
+				set (USE_CXX_STD_APP ${STDCXX})
+				message(STATUS "C++ STANDARD: library: C++${STDCXX}, apps disabled (examples will follow C++${STDCXX})")
+			else()
+				set (USE_CXX_STD_APP "")
+				message(STATUS "C++ STANDARD: library: C++${STDCXX}, but apps still at least C++11")
+			endif()
 		elseif (FORCE_CXX_STANDARD_GNUONLY)
 			# CMake is too old to handle CMAKE_CXX_STANDARD,
 			# use bare GNU options.
@@ -805,10 +807,10 @@ macro(srt_set_stdcxx targetname spec)
 	if (NOT "${stdcxxspec}" STREQUAL "")
 		if (FORCE_CXX_STANDARD_GNUONLY)
 			target_compile_options(${targetname} PRIVATE -std=c++${stdcxxspec})
-			#message(STATUS "C++ STD: ${targetname}: forced C++${stdcxxspec} standard - GNU option: -std=c++${stdcxxspec}")
+			message(STATUS "C++ STD: ${targetname}: forced C++${stdcxxspec} standard - GNU option: -std=c++${stdcxxspec}")
 		else()
 			set_target_properties(${targetname} PROPERTIES CXX_STANDARD ${stdcxxspec})
-			#message(STATUS "C++ STD: ${targetname}: forced C++${stdcxxspec} standard - portable way")
+			message(STATUS "C++ STD: ${targetname}: forced C++${stdcxxspec} standard - portable way")
 		endif()
 	else()
 		message(STATUS "APP: ${targetname}: using default C++ standard")

docs/BuildOptions.md

@@ -303,7 +303,12 @@ when it is expected to be revived.
 
 **`--use-c++-std=<standard>`**
 
+Enforce using particular C++ standard when compiling. When using this option
+remember that:
 
+* Allowed values are: 98, 03, 11, 14, 17 and 20
+* If you use 98/03 and `--enable-apps`, apps will be still using C++11
+* This option is only supported on GNU and Clang compilers (will be ignored on others)
 
 **`--use-gnustl`**
 

docs/DevelopersGuide.md

@@ -77,12 +77,36 @@ Please see the following document for `configure` usage: [BuildOptions.md](./Bui
 
 The build output is in the `_build` directory. The following applications can be found there.
 
-* `srt-live-transmit` — A sample application to transmit a live stream from source medium (UDP/SRT/`stdin`)
+* `srt-live-transmit` - A sample application to transmit a live stream from source medium (UDP/SRT/`stdin`)
 to the target medium (UDP/SRT/`stdout`). See [srt-live-transmit.md](./srt-live-transmit.md) for more info.
-* `srt-file-transmit` — A sample application to transmit files with SRT.
-* `srt-tunnel` — A sample application to set up an SRT tunnel for TCP traffic. See [srt-tunnel.md](./srt-tunnel.md) for more info.
+* `srt-file-transmit` - A sample application to transmit files with SRT.
+* `srt-tunnel` - A sample application to set up an SRT tunnel for TCP traffic. See [srt-tunnel.md](./srt-tunnel.md) for more info.
 * `tests-srt` - unit testing application.
 
+## Language standard requirements
+
+The following conventions for the language standard are used in this project:
+
+1. The SRT library requires C++03 (also known as C++98) standard.
+2. The examples (to be enabled in cmake by `-DENABLE_EXAMPLES=1`) require either C++03 or C89 standard.
+3. The following require C++11 standard:
+   * demo applications
+   * testing applications (to be enabled in cmake by `-DENABLE_TESTING=1`)
+   * unit tests (to be enabled in cmake by `-DENABLE_UNITTESTS=1`)
+
+Note that C++11 standard will be enforced if you have enabled applications
+and haven't specified the C++ standard explicitly. When you have an old compiler
+that does not support C++11 and you want to compile as many parts as possible,
+the simplest way is to use the following options (in cmake):
+
+```
+-DENABLE_APPS=0 -DUSE_CXX_STD=03 -DENABLE_EXAMPLES=1
+```
+
+Note also that there are several other options that, when enabled, may require
+that the SRT library be compiled using C++11 standard (`-DENABLE_STDCXX_SYNC=1`
+for example).
+
 ## Project Structure
 
 The SRT installation has the following folders: