allow literal string only

[superbonzo]

as other said it seems impossible with the use of the sole type system ( unless compiler intrisics exist enabling static storage detection ... );

maybe you can use the preprocessor somehow, something like

#define LITERAL(s) Foo::please_dont_use_me(""s"")
#define LITERAL_TYPE please_dont_use_me

class Foo
{
public:
	struct please_dont_use_me
	{
		explicit please_dont_use_me( const char* str ): value(str) {}

		const char* value;
	};

	Foo( LITERAL_TYPE str ): str_( str.value ) {}

private:
	const char* str_;
};

Foo x( LITERAL("ok") );

in this way, if you construct a Foo with something that has not the form LITERAL("...") an error should occur ( more specifically, it seems improbable someone accidentally instances a Foo with a Foo :: please_dont_use_me object ... ).

EDIT: thinking about it, the following might be more expressive ( but roughly equivalent ):

#define LITERAL(s) Foo::please_dont_use_me(""s"")

class Foo
{
	struct literal
	{
		explicit literal( const char* str ): value(str) {}

		const char* value;
	};

public:
	typedef literal please_dont_use_me;

	Foo( literal str ): str_( str.value ) {}

private:
	const char* str_;
};

Foo x( LITERAL("ok") );

BTW, note that it wouldn't protect you against expressions like "Foo x( LITERAL("ok"+"oops") );" and similar ...