How to format a string according to the formatting method entered by user in c ?
For example, here is an integer:
int x = 100;
If a user inputs a formatting method (a string):
"%x"，
I want to output: 0x40.

And another user inputs a formatting string:
%.4f,
I want it output:100.0000.
How to do it?

CodePudding user response：

printf accepts any string as its first argument, not just string literals. Just be careful that the format must match the arguments. That makes this rather error-prone. E.g. specifying %d and passing char is undefined behaviour.

int main(){
    std::string user_format = "%x";

    int x = 100;

    std::printf(user_format.c_str(),x);
}

std::hex and std::setprecision modifiers might be more suited for this task.

CodePudding user response：

I see the following two concepts:

1

• read in the formatting string from the user
• reject anything with more than one format specifier
• reject anything with wrong format specifier for the type of the variable you want to output (using a float specifier for an integer for example would be wrong; just mentioning it specifically because your example looks like this....)
• reject anything with the wrong format specifier for the width of the variable (same as above, just another detail)
• reject wrong specifiers which do not attempt to output the variable (there are some fancy/dagerous things which could otherwise be done with a user-provided string...)
• remove anything but the format specifier (assuming you mean to not allow "decoration")
• either reject above by removing it and use the remainder
• or reject by giving an error message and asking for another input
• use the string for output

2

• provide user with a choice of predefined safe and applicable formatting options

For security and reliability reasons I would probably go with option 2.