aave-protocol-v2/contracts/libraries/openzeppelin-upgradeability/Initializable.sol

// SPDX-License-Identifier: agpl-3.0
pragma solidity >=0.4.24 <0.7.0;

/**
 * @title Initializable
 *
 * @dev Helper contract to support initializer functions. To use it, replace
 * the constructor with a function that has the `initializer` modifier.
 * WARNING: Unlike constructors, initializer functions must be manually
 * invoked. This applies both to deploying an Initializable contract, as well
 * as extending an Initializable contract via inheritance.
 * WARNING: When used with inheritance, manual care must be taken to not invoke
 * a parent initializer twice, or ensure that all initializers are idempotent,
 * because this is not dealt with automatically as with constructors.
 */
contract Initializable {
    /**
   * @dev Indicates that the contract has been initialized.
   */
    bool private initialized;

    /**
   * @dev Indicates that the contract is in the process of being initialized.
   */
    bool private initializing;

    /**
   * @dev Modifier to use in the initializer function of a contract.
   */
    modifier initializer() {
        require(initializing || isConstructor() || !initialized, "Contract instance has already been initialized");

        bool isTopLevelCall = !initializing;
        if (isTopLevelCall) {
            initializing = true;
            initialized = true;
        }

        _;

        if (isTopLevelCall) {
            initializing = false;
        }
    }

    /// @dev Returns true if and only if the function is running in the constructor
    function isConstructor() private view returns (bool) {
        // extcodesize checks the size of the code stored in an address, and
        // address returns the current address. Since the code is still not
        // deployed when running a constructor, any checks on its code size will
        // yield zero, making it an effective way to detect if a contract is
        // under construction or not.
        uint256 cs;
        //solium-disable-next-line
        assembly {
            cs := extcodesize(address())
        }
        return cs == 0;
    }

    // Reserved storage space to allow for layout changes in the future.
    uint256[50] private ______gap;
}