Classic VB - What is datatype coercion?

[Merri]

Datatype coercion is a feature of Visual Basic that can't be found in many other programming languages. It allows you to use different datatypes against each other without you bothering much about making sure the datatypes can actually be compared against each other.

For example, you can do this:

Dim strTemp As String
 
    ' coercion happens here: 500 is automatically converted to "500"
    strTemp = 500
 
    Dim intTemp As Integer
    intTemp = 500
 
    ' coercion happens here again: strTemp is now automatically converted to an Integer value
    If strTemp = intTemp Then MsgBox "Datatype coercion occurred.", vbInformation

This makes using of the programming language far easier, but it has it's downfalls as well. Sometimes there can be situatations when coercion doesn't know how to work and you get an error:

Dim strTemp As String
    Dim intTemp As Integer
    intTemp = 500
    If strTemp = intTemp Then MsgBox "Datatype coercion occurred.", vbInformation

If you try running this code, you get error 13: type mismatch, as an empty string cannot be converted into a number.


What you should do is to make sure you are comparing same datatypes against each other. Using the same datatypes against each other is also much faster as VB doesn't need to do coercion in that case. The resulting compiled code will also be shorter and much more efficient as there will be no need for the extra conversion code (which is invisible to you).

So, we are comparing numbers here. Lets make the string a number:

If Val(strTemp) = intTemp Then ...

Please note that coercion still takes place here: Val() returns a double value. You could also add CInt() surrounding the Val(). You can't use CInt directly, because strTemp isn't counted as a number and thus CInt would give you the same type mismatch error when the string is empty.

Although it would be the best if you didn't use a string in the first place. You could plan things like this:

• visual layer: convert to strings when displaying something to the user and convert values given by user back to proper datatype when done with the visual side
• "under the hood" layer: everything possible is handled as numbers using the best suited datatype for each case (except native string data, of course)
• output layer: if you need to save, then convert to the format you wish to use for your save files

This will prevent getting errors and weird bugs, and makes sure your code is efficient.


A quick guide to the most used datatypes

Integers
Byte = 8 bits unsigned integer, 0 to 255
Integer = 16 bits signed integer, -32768 to 32767
Long = 32 bits signed integer, -2147483648 to 2147483647
Currency = 64 bits hacked signed integer, can have four decimal numbers

Integer numbers are always accurate.

Floating point
Single = 32 bits floating point
Double = 64 bits floating point

Floating point numbers aren't perfectly accurate, but give a possibility to have a lot of decimals. Never use for money calculations!

Special
Boolean = 16 bits, True or False
Date = 64 bits floating point (actually a Double)
String = 32 bits + byte length of string + null character