Add another example for generators

examples/301-Gen-MapTypeConversion.cpp

@@ -0,0 +1,54 @@
+// 301-Gen-MapTypeConversion.cpp
+// Shows how to use map to modify generator's return type.
+
+// TODO
+
+#include <catch2/catch.hpp>
+
+#include <string>
+#include <sstream>
+
+// Returns a line from a stream. You could have it e.g. read lines from
+// a file, but to avoid problems with paths in examples, we will use
+// a fixed stringstream.
+class LineGenerator : public Catch::Generators::IGenerator<std::string> {
+    std::string m_line;
+    std::stringstream m_stream;
+public:
+    LineGenerator() {
+        m_stream.str("1\n2\n3\n4\n");
+        if (!next()) {
+            throw Catch::GeneratorException("Couldn't read a single line");
+        }
+    }
+
+    std::string const& get() const override {
+        return m_line;
+    }
+    
+    bool next() override {
+        return !!std::getline(m_stream, m_line);
+    }
+};
+
+// This helper function provides a nicer UX when instantiating the generator
+// Notice that it returns an instance of GeneratorWrapper<std::string>, which
+// is a value-wrapper around std::unique_ptr<IGenerator<std::string>>.
+Catch::Generators::GeneratorWrapper<std::string> lines(std::string /* ignored for example */) {
+    return Catch::Generators::GeneratorWrapper<std::string>(
+        std::unique_ptr<Catch::Generators::IGenerator<std::string>>(
+            new LineGenerator()
+        )
+    );
+}
+
+
+
+TEST_CASE("filter can convert types inside the generator expression", "[example][generator]") {
+    auto num = GENERATE(map<int>([](std::string const& line) { return std::stoi(line); },
+                                 lines("fake-file")));
+                                 
+    REQUIRE(num > 0);
+}
+
+// Compiling and running this file will result in 4 successful assertions