add piston nix pkg for gcc-c++

runtimes/default.nix

@@ -46,4 +46,5 @@ args: {
     "groovy" = import ./groovy.nix args;
     "go" = import ./go.nix args;
     "gcc-c" = import ./gcc-c.nix args;
+    "gcc-c++" = import ./gcc-c++.nix args;
 }

runtimes/gcc-c++.nix

@@ -0,0 +1,42 @@
+{pkgs, piston, ...}:
+let
+    pkg = pkgs.gcc;
+in piston.mkRuntime {
+    language = "c++";
+    version = pkg.version;
+    runtime = "gcc";
+
+    aliases = [
+        "cpp"
+        "g++"
+    ];
+
+    compile = ''
+        rename 's/$/\.cpp/' "$@" # Add .cpp extension
+        ${pkg}/bin/g++ -std=c++17 *.cpp
+    '';
+
+    run = ''
+        shift
+        ./a.out "$@"
+    '';
+
+    tests = [
+        (piston.mkTest {
+            files = {
+                "test.cpp" = ''
+                    #include <iostream>
+
+                    int main(void) {
+                        printf("OK");
+                        return 0;
+                    }
+                '';
+            };
+            args = [];
+            stdin = "";
+            packages = [];
+            main = "test.cpp";
+        })
+    ];
+}