Considering that the very first line of a solidity smart contract is something like:
pragma solidity ^0.4.21
What does the caret really mean? The short answer is the caret or top hat (^) means the code will be compatible with compiler version from 0.4.21 to 0.5.0.
This sounds easy to understand but it doesn’t actually tell the whole story. To really understand it, means learning about the concept of Semantic Versioning. In short, software versioning generally follows the format major.minor.revision. The caret is only one symbol out of various others such as ~ (tilde), * (wild card), || (logical or) and a bunch of typical ones such as >, >=, < and <=.
The real definition of ^ is: “Allows changes that do not modify the left-most non-zero digit in the major.minor.version tuple format.“ This definition explains the following examples:
^1.2.3: >=1.2.3 and <2.0.0
^0.2.3: >=0.2.3 and <0.3.0
^0.0.3: >=0.0.3 and <0.0.4
Tilde is interesting. The definition is: “Allows patch-level changes if a minor version is specified on the comparator. Allows minor-level changes if not.” which again is explained by these examples:
~1.2.3: >=1.2.3 and <1.3.0
~0.2.3: >=0.2.3 and <0.3.0
~1.2:>=1.2.0 and <1.3.0 (Same as 1.2.x)
~1: >=1.0.0 <2.0.0 (Same as 1.x)
Learning the fundamentals provides a deeper understanding and also helps the recall as well.
This blog was inspired by: https://www.youtube.com/watch?v=KiTOycVbgcI