Report missing keys

README.md

@@ -210,6 +210,32 @@ mstch::config::escape = [](const std::string& str) -> std::string {
 };
 ```
 
+
+### Handling missing keys
+
+By default, mstch does not consider missing keys a problem but simply ignores
+them. If the client expects all keys referenced by a template to exist, mstch
+can be configured to throw an exception if a key is not found:
+
+```c++
+#include <iostream>
+#include <mstch/mstch.hpp>
+#include <string>
+
+int main() {
+  mstch::config::throw_on_missing_key = true;
+  const std::string view{"{{a-missing-key}}"};
+  mstch::map context{{"an-existing-key", std::string{"a value"}}};
+  try {
+    std::cout << mstch::render(view, context) << std::endl;
+  } catch (const mstch::key_not_found& e) {
+    std::cout << "Failed to render the template: " << e.what() << std::endl;
+  }
+  return 0;
+}
+```
+
+
 ## Requirements
 
  - A C++ compiler with decent C++11 support. Currently tested with:

include/mstch/mstch.hpp

@@ -5,13 +5,31 @@
 #include <string>
 #include <memory>
 #include <functional>
+#include <exception>
 
 #include <boost/variant.hpp>
 
 namespace mstch {
 
+class key_not_found : public std::exception {
+ public:
+  key_not_found(const std::string& name):
+    description(std::string{"The key '"} + name + "' is missing")
+  {
+  }
+
+  const char* what() const noexcept override
+  {
+    return description.c_str();
+  }
+
+private:
+  const std::string description;
+};
+
 struct config {
   static std::function<std::string(const std::string&)> escape;
+  static bool throw_on_missing_key;
 };
 
 namespace internal {

src/mstch.cpp

@@ -6,6 +6,7 @@
 using namespace mstch;
 
 std::function<std::string(const std::string&)> mstch::config::escape;
+bool mstch::config::throw_on_missing_key = false;
 
 std::string mstch::render(
     const std::string& tmplt,

src/render_context.cpp

@@ -43,6 +43,8 @@ const mstch::node& render_context::find_node(
     for (auto& node: current_nodes)
       if (visit(has_token(token), *node))
         return visit(get_token(token, *node), *node);
+  if (mstch::config::throw_on_missing_key)
+    throw key_not_found{token};
   return null_node;
 }
 

test/CMakeLists.txt

@@ -54,6 +54,9 @@ add_executable(mstch_test test_main.cpp)
 target_link_libraries(mstch_test mstch)
 add_dependencies(mstch_test test_data_hpp specs_data_hpp)
 
+list(APPEND tests "missing_key_is_ignored")
+list(APPEND tests "missing_key_is_reported")
+
 foreach(test ${tests})
   add_test(NAME ${test} COMMAND mstch_test ${test})
 endforeach(test)

test/test_main.cpp

@@ -153,3 +153,17 @@ SPECS_TEST(inverted)
 SPECS_TEST(partials)
 SPECS_TEST(sections)
 SPECS_TEST(lambdas)
+
+TEST_CASE("missing_key_is_ignored") {
+  mstch::config::throw_on_missing_key = false;
+  const std::string view{"{{a-non-existing-key}}"};
+  mstch::map context{{"an-existing-key", std::string{"a value"}}};
+  REQUIRE_NOTHROW(mstch::render(view, context));
+}
+
+TEST_CASE("missing_key_is_reported") {
+  mstch::config::throw_on_missing_key = true;
+  const std::string view{"{{a-missing-key}}"};
+  mstch::map context{{"an-existing-key", std::string{"a value"}}};
+  REQUIRE_THROWS_AS(mstch::render(view, context), mstch::key_not_found);
+}