Merge PR #173 from epsilon-phase into dev

Author: zauguin

README.md

@@ -370,6 +370,7 @@ This does not happen if you use a `variant`.
 If the `variant` does an alternative of the same value type, an `mismatch` exception will be thrown.
 The value types are NULL, integer, real number, text and BLOB.
 To support all possible values, you can use `variant<nullptr_t, sqlite_int64, double, string, vector<char>`.
+It is also possible to use a variant with `std::monostate` in order to catch null values.
 
 Errors
 ----

hdr/sqlite_modern_cpp/type_wrapper.h

@@ -167,6 +167,24 @@ namespace sqlite {
 		sqlite3_result_null(db);
 	}
 
+#ifdef MODERN_SQLITE_STD_VARIANT_SUPPORT
+	template<>
+	struct has_sqlite_type<std::monostate, SQLITE_NULL, void> : std::true_type {};
+
+	inline int bind_col_in_db(sqlite3_stmt* stmt, int inx, std::monostate) {
+		return sqlite3_bind_null(stmt, inx);
+	}
+	inline void store_result_in_db(sqlite3_context* db, std::monostate) {
+		sqlite3_result_null(db);
+	}
+	inline std::monostate get_col_from_db(sqlite3_stmt* stmt, int inx, result_type<std::monostate>) {
+		return std::monostate();
+	}
+	inline std::monostate get_val_from_db(sqlite3_value *value, result_type<std::monostate>) {
+		return std::monostate();
+	}
+#endif
+
 	// str_ref
 	template<>
 	struct has_sqlite_type<std::string, SQLITE3_TEXT, void> : std::true_type {};

tests/variant.cc

@@ -29,4 +29,29 @@ TEST_CASE("std::variant works", "[variant]") {
     db << "SELECT 0.0" >> v;
     REQUIRE(std::get<2>(v)); 
 }
+
+TEST_CASE("std::monostate is a nullptr substitute", "[monostate]") {
+    database db(":memory:");
+    db << "CREATE TABLE foo (a);";
+
+    std::variant<std::monostate,std::string> v;
+    v=std::monostate();
+    db << "INSERT INTO foo VALUES (?)" << v;
+    db << "INSERT INTO foo VALUES (?)" << "This isn't a monostate!";
+
+    bool found_null   = false,
+         found_string = false;
+
+    db << "SELECT * FROM foo"
+    	 >> [&](std::variant<std::monostate, std::string> z) {
+      if(z.index() == 0) {
+        found_null = true;
+      } else {
+        found_string = true;
+      }
+    };
+    REQUIRE((found_null && found_string));
+    db << "SELECT NULL" >> v;
+    REQUIRE(v.index() == 0);
+}
 #endif