Merge pull request #3268 from memsharded/feature/cpp_info

CppInfo(self)/aggregate new public interface

reference/tools.rst

@@ -38,3 +38,4 @@ Contents:
    tools/layout
    tools/intel
    tools/android
+   tools/cpp_info

reference/tools/cpp_info.rst

@@ -0,0 +1,50 @@
+.. _conan_tools_cpp_info:
+
+conan.tools.CppInfo
+===================
+
+The ``CppInfo`` class represents the basic C++ usage information of a given package, like the ``includedirs``, ``libdirs``, library names, etc. It is the information that the consumers of the package need in order to be able to find the headers and link correctly with the libraries.
+
+The ``self.cpp_info`` object in the ``package_info()`` is a ``CppInfo`` object, so in most cases it will not be necessary to explicitly instantiate it, and using it as explained in :ref:`the package_info()<reference_conanfile_methods_package_info>` section would be enough.
+
+
+This section describes the other, advanced uses cases of the ``CppInfo``.
+
+Aggregating information in custom generators
+--------------------------------------------
+
+.. include:: ../../common/experimental_warning.inc
+
+Some generators, like the built-in ``NMakeDeps``, contains the equivalent to this code, that collapses all information from all dependencies into one single ``CppInfo`` object that aggregates all the information
+
+.. code-block:: python
+
+  from conan.tools import CppInfo
+  
+  ...
+  
+  def generate(self):
+      aggregated_cpp_info = CppInfo(self)
+      deps = self.dependencies.host.topological_sort
+      deps = [dep for dep in reversed(deps.values())]
+      for dep in deps:
+          # We don't want independent components management, so we collapse
+          # the "dep" components into one CppInfo called "dep_cppinfo"
+          dep_cppinfo = dep.cpp_info.aggregated_components()
+          # Then we merge and aggregate this dependency "dep" into the final result
+          aggregated_cpp_info.merge(dep_cppinfo)
+      
+      aggregated_cpp_info.includedirs  # All include dirs from all deps, all components
+      aggregated_cpp_info.libs  # All library names from all deps, all components
+      aggregated_cpp_info.system_libs # All system-libs from all deps
+      ....
+      # Creates a file with this information that the build system will use
+
+
+This aggregation could be useful in cases where the build system cannot easily use independent dependencies or components. For example ``NMake`` or ``Autotools`` mechanism to provide dependencies information would be via ``LIBS``, ``CXXFLAGS`` and similar variables. These variables are global, so passing all the information from all dependencies is the only possibility.
+
+The public documented interface (besides the defined one in :ref:`the package_info()<reference_conanfile_methods_package_info>`) is:
+
+- ``CppInfo(conanfile)``: Constructor. Receives a ``conanfile`` as argument, typically ``self``
+- ``aggregated_components()``: return a new ``CppInfo`` object resulting from the aggregation of all the components
+- ``merge(other_cppinfo: CppInfo)``: modifies the current ``CppInfo`` object, updating it with the information of the parameter ``other_cppinfo``, allowing to aggregate information from multiple dependencies.