Added lambda with capture group support to USRs

Closes #10

tests/tetests.cpp

@@ -2052,6 +2052,20 @@ TEST_CASE("Unknown symbol resolve funct pointer purge resolved 2", "[usr]")
     CHECK(parser.evaluate("id.temperature") == 53);
     }
 
+TEST_CASE("Unknown symbol resolve lambda with capture", "[usr]")
+    {
+    std::string str("id.temperature < 51");
+    double temperature = 49.0;
+    te_parser parser;
+    parser.set_unknown_symbol_resolver([&](std::string_view symbol)
+        {
+        return temperature += 1.0;
+        }, false);
+    CHECK(parser.compile(str));        // 50
+    CHECK_FALSE(parser.evaluate(str)); // 51
+    CHECK_FALSE(parser.evaluate(str)); // 52
+    CHECK(parser.evaluate("id.temperature") == 53);
+    }
 
 TEST_CASE("Unknown symbol resolve 1 param purged", "[usr]")
     {

tinyexpr.h

@@ -67,6 +67,7 @@
 #include <cctype>
 #include <set>
 #include <utility>
+#include <functional>
 
 class te_expr;
 
@@ -90,9 +91,9 @@ using te_confun6 = double (*)(const te_expr*, double, double, double, double, do
 using te_confun7 = double (*)(const te_expr*, double, double, double, double, double, double, double);
 
 // functions for unknown symbol resolution
-using te_usr_noop = void (*)();
-using te_usr_fun0 = double (*)(std::string_view);
-using te_usr_fun1 = double (*)(std::string_view, std::string&);
+using te_usr_noop = std::function<void ()>;
+using te_usr_fun0 = std::function<double (std::string_view)>;
+using te_usr_fun1 = std::function<double (std::string_view, std::string&)>;
 
 using te_usr_variant_type = std::variant<te_usr_noop, te_usr_fun0, te_usr_fun1>;