I am writing a program. It needs user to type in the input. eg. 2300.00

how can I block the character. In other words, the user cannot type in character and only accept numbers.

You can't...you'll just have to let him input the data and then check whether it's valid or not.

so how can i check it valid or not

If you just want to ignore some characters, you can use the method:
istream& istream::ignore ([int N] [, int DELIM]).

For more info, type:
info iostream streams istream "Istream Housekeeping"

If you want to actually find the invalid char's, you'll have to read a string and scan it yourself using a loop. For reading strings choose a method from this info page:

info iostream streams istream "string input"

If the info-doc's are not on your system you can (depending on your linux distribution) install the "libstdc++2.10-dev" (for Debian-testing), or similar package.

is there a way to have it error and tell the user to try again instead of crashing when they enter an char for an int(cause in menu programs it gets annoying to go through the whole series of options to get back to where you want.

Don't accept input as a number. Accept it as a string. Then you can check the characters in the string one by one against ASCII values. If all of the characters in the string have ASCII values of number 0 to 9, the input is valid and convert the whole string to the numeric and process it. Otherwise, give error message.

write your own edit/input class