[Ethereum] How to use openzeppelin contracts for a specific version

It's a safety feature.

It might help to think of the solc compiler as a possible source of defects. It's a way of stating the version of compiler that was used in development and testing.

Consider a situation in which a team has conducted painstaking testing at great expense. Then, a series of unfortunate circumstances leads to a compile using a newer compiler. They put it into production with the mistaken belief that it was thoroughly tested, but in reality, it's different and broken. Oops!

The compiler will throw an error if it doesn't match what the source file asks for.

pragma solidity 0.4.11; // version 0.4.11. Nothing else will do.

pragma solidity ^0.4.11; // version 0.4.11 or newer 

In my opinion, the ^ notation is convenient for informal development. The precise notation is required before extensive testing or release.

It basically says "this code was compiled with 0.4.11" and people can compile the source themselves and see that it matches exactly. Implicitly, if the version that was tested was compiled with something else, then "this" bytecode wasn't really tested at all.

Hope it helps.

Current version of open-zeppelin (2.0.0) relies on solc version 0.4.24.

Current version of truffle (5.0.0) relies on solc version 0.5.0.

Between solc 0.4.x and solc 0.5.x there are breaking changes.

Previous version of truffle (4.1.14) relies on solc version 0.4.24.

So in order to use open-zeppelin in conjunction with truffle, set in your package.json file:

"devDependencies": {
  "truffle": "4.1.14",
  "openzeppelin-solidity": "2.0.0",
  ...
}

Alternatively, in truffle 5.0.0 onward, you can choose the desired version of solc.

For example, in order to configure truffle to use solc 0.4.24, add this in truffle-config.js:

compilers: {
    solc: {
        version: "0.4.24"
    }
}

See more details in Truffle 5.0.0 release notes for how to bring your own compiler.