How to call the right constructor of a template type?

Use braced-init-list(or uniform-initiation) to initlize the instance of Class.

Class<std::string> a{ std::string("abc") };  // works
Class<std::string> b{ "abc" };               // also works

Class<std::string> b = "abc";

is copy-initialization. It doesn't work because it would involve two user-defined conversions:

• from const char* to std::string,
• from std::string to Class<std::string>.

But at most one is allowed.

When you write

Class<std::string> b("abc");
// or
Class<std::string> b{"abc"};

you use direct-initialization. It works because now only one user-defined conversion is used:

• from const char* to std::string.

Use direct initialization:

Class<std::string> b("abc");