fixed std::string builder

include/fast_strcat/string_builder.hpp

@@ -0,0 +1,34 @@
+#include <iostream>
+#include <string_view>
+#include <tuple>
+#include <string>
+
+template<typename... Args>
+struct string_builder
+{
+    std::tuple<Args&...> vals;
+
+    template <typename T>
+    constexpr auto operator<<(T& v) && -> string_builder<Args..., T>
+    {
+        return {std::tuple_cat(vals, std::tuple<T&>{v})};
+    }
+
+    constexpr operator std::string() &&
+    {
+        auto len = std::size_t{0};
+        std::apply([&](auto&... x){  (len += ... += std::string_view{x}.length());  } , vals);
+
+        auto res = std::string{};
+        res.reserve(len);
+
+        std::apply([&](auto&... x){  (res += ... += x);  } , vals);
+
+        return res;
+    }
+};
+
+constexpr auto build_string()
+{
+    return string_builder<>{};
+}

test/main.cpp

@@ -1,8 +1,13 @@
 #include <fast_strcat/strcat.hpp>
 #include <fast_strcat/string_builder.hpp>
 
+#include <cassert>
 
 int main()
 {
+    std::string test2 = "test2";
+    const char* test4 = "test4";
+    std::string test = build_string() << "test1 " << test2 << " test3 " << test4;
+    assert(test == "test1 test2 test3 test4");
 	return 0;
 }