Added test and documentation

Author: zauguin

README.md

@@ -304,6 +304,25 @@ sqlite::sqlite_exception has a get_code() member function to get the SQLITE3 err
 	   catch(sqlite::exceptions::constraint e) {  } */
 ```
 
+Custom SQL functions
+----
+
+To extend SQLite with custom functions, you just implement them in C++:
+
+```c++
+  database db(":memory:");
+  db.define("tgamma", [](double i) {return std::tgamma(i);});
+  db << "CREATE TABLE numbers (number INTEGER);";
+
+  for(auto i=0; i!=10; ++i)
+    db << "INSERT INTO numbers VALUES (?);" << i;
+
+  db << "SELECT number, tgamma(number+1) FROM numbers;" >> [](double number, double factorial) {
+    cout << number << "! = " << factorial << '\n';
+  };
+```
+
+
 NDK support
 ----
 Just Make sure you are using the full path of your database file :

tests/functions.cc

@@ -0,0 +1,65 @@
+#include <iostream>
+#include <cstdlib>
+#include <cmath>
+#include <sqlite_modern_cpp.h>
+using namespace  sqlite;
+using namespace std;
+
+int main()
+{
+	try
+	{
+		database db(":memory:");
+
+    db.define("my_new_concat", [](std::string i, std::string j) {return i+j;});
+    db.define("my_new_concat", [](std::string i, std::string j, std::string k) {return i+j+k;});
+    db.define("add_integers", [](int i, int j) {return i+j;});
+		std::string test1, test3;
+		int test2 = 0;
+		db << "select my_new_concat('Hello ','world!')" >> test1;
+		db << "select add_integers(1,1)" >> test2;
+		db << "select my_new_concat('a','b','c')" >> test3;
+		
+		if(test1 != "Hello world!" || test2 != 2 || test3 != "abc") {
+      cout << "Wrong result\n";
+		  exit(EXIT_FAILURE);
+		}
+
+		db.define("my_count", [](int &i, int) {++i;}, [](int &i) {return i;});
+		db.define("my_concat_aggregate", [](std::string &stored, std::string current) {stored += current;}, [](std::string &stored) {return stored;});
+		db << "create table countable(i, s)";
+		db << "insert into countable values(1, 'a')";
+		db << "insert into countable values(2, 'b')";
+		db << "insert into countable values(3, 'c')";
+		db << "select my_count(i) from countable" >> test2;
+		db << "select my_concat_aggregate(s) from countable order by i" >> test3;
+
+		if(test2 != 3 || test3 != "abc") {
+		  cout << "Wrong result\n";
+		  exit(EXIT_FAILURE);
+		}
+
+    db.define("tgamma", [](double i) {return std::tgamma(i);});
+    db << "CREATE TABLE numbers (number INTEGER);";
+
+    for(auto i=0; i!=10; ++i)
+      db << "INSERT INTO numbers VALUES (?);" << i;
+
+    db << "SELECT number, tgamma(number+1) FROM numbers;" >> [](double number, double factorial) {
+      cout << number << "! = " << factorial << '\n';
+    };
+	}
+	catch(sqlite_exception e)
+	{
+		cout << "Unexpected error " << e.what() << endl;
+		exit(EXIT_FAILURE);
+	}
+	catch(...)
+	{
+		cout << "Unknown error\n";
+		exit(EXIT_FAILURE);
+	}
+
+	cout << "OK\n";
+	exit(EXIT_SUCCESS);
+}