fs/Traits: add Apply()

src/fs/AllocatedPath.hxx

@@ -124,6 +124,11 @@ public:
 			     b.value.c_str(), b.value.size());
 	}
 
+	gcc_pure
+	static AllocatedPath Apply(Path base, Path path) noexcept {
+		return Traits::Apply(base.c_str(), path.c_str());
+	}
+
 	/**
 	 * Convert a C string that is already in the filesystem
 	 * character set to a #Path instance.

src/fs/Traits.cxx

@@ -144,6 +144,20 @@ PathTraitsFS::Relative(const_pointer_type base, const_pointer_type other) noexce
 	return RelativePathImpl<PathTraitsFS>(base, other);
 }
 
+PathTraitsFS::string
+PathTraitsFS::Apply(const_pointer_type base, const_pointer_type path) noexcept
+{
+	// TODO: support the Windows syntax (absolute path with or without drive, drive with relative path)
+
+	if (base == nullptr)
+		return path;
+
+	if (IsAbsolute(path))
+		return path;
+
+	return Build(base, path);
+}
+
 PathTraitsUTF8::string
 PathTraitsUTF8::Build(const_pointer_type a, size_t a_size,
 		      const_pointer_type b, size_t b_size) noexcept

src/fs/Traits.hxx

@@ -158,6 +158,14 @@ struct PathTraitsFS {
 	static string Build(const_pointer_type a, const_pointer_type b) noexcept {
 		return Build(a, GetLength(a), b, GetLength(b));
 	}
+
+	/**
+	 * Interpret the given path as being relative to the given
+	 * base, and return the concatenated path.
+	 */
+	gcc_pure
+	static string Apply(const_pointer_type base,
+			    const_pointer_type path) noexcept;
 };
 
 /**