Std::string size different with thirdparty

I have a thirdparty lib which have an operation with #pragma execution_character_set(“utf-8”). And there is an Interface has a param is std::string. the size of empty string is 40 which in my UE4 Project the size is 32.How can i set my project std::string size such as the thirdparty as the same?
I trid to add the same compile operate into my project, but it throw an error which is error C3437