SWC-131/存在未使用的变量

Solidity中允许存在未使用的变量，并且它们不会带来直接的安全问题。但最好的做法是尽量避免使用它们，因为：

导致计算量增加（以及不必要的gas消耗）
表示错误或数据结构不正确，通常表示代码质量不佳
导致代码噪音并降低代码的可读性

CWE错误分类

CWE-1164：不相关的代码

整改方案

从代码库中删除所有未使用的变量。

参考文献

示例合约

unused_state_variables.sol

pragma solidity >=0.5.0;
pragma experimental ABIEncoderV2;

import "./base.sol";

contract DerivedA is Base {
    // i is not used in the current contract
    A i = A(1);

    int internal j = 500;

    function call(int a) public {
        assign1(a);
    }

    function assign3(A memory x) public returns (uint) {
        return g[1] + x.a + uint(j);
    }

    function ret() public returns (int){
        return this.e();

    }

}

used_state_variables.yaml

description: Presence of unused state variables
issues:
- id: SWC-131
  count: 4
  locations:
  - bytecode_offsets: {}
    line_numbers:
      unused_state_variables.sol: [8]
  - bytecode_offsets: {}
    line_numbers:
      base.sol: [10]
  - bytecode_offsets: {}
    line_numbers:
      base.sol: [11]
  - bytecode_offsets: {}
    line_numbers:
      base.sol: [17]

unused_state_variables.yaml

pragma solidity >=0.5.0;
pragma experimental ABIEncoderV2;

import "./base_fixed.sol";

contract DerivedA is Base {

    int internal j = 500;

    function call(int a) public {
        assign1(a);
    }

    function assign3(A memory x) public returns (uint) {
        return g[1] + x.a + uint(j);
    }

    function ret() public returns (int){
        return this.e();

    }

}

used_state_variables_fixed.yaml

description: Presence of unused state variables
issues:
- id: SWC-131
  count: 0
  locations: []

unused_variables.sol

pragma solidity ^0.5.0;

contract UnusedVariables {
    int a = 1;

    // y is not used
    function unusedArg(int x, int y) public view returns (int z) {
        z = x + a;  
    }

    // n is not reported it is part of another SWC category
    function unusedReturn(int x, int y) public pure returns (int m, int n, int o) {
        m = y - x;
        o = m/2;
    }

    // x is not accessed 
    function neverAccessed(int test) public pure returns (int) {
        int z = 10;

        if (test > z) {
            // x is not used
            int x = test - z;

            return test - z;
        }

        return z;
    }

    function tupleAssignment(int p) public returns (int q, int r){
        (q, , r) = unusedReturn(p,2);

    }


}

unused_variables.yaml

description: Presence of unused variables
issues:
- id: SWC-131
  count: 2
  locations:
  - bytecode_offsets: {}
    line_numbers:
      unused_variables.sol: [7]
  - bytecode_offsets: {}
    line_numbers:
      unused_variables.sol: [23]

unused_variables_fixed.sol

pragma solidity ^0.5.0;

contract UnusedVariables {
    int a = 1;

    function unusedArg(int x) public view returns (int z) {
        z = x + a;  
    }

    // n is not reported it is part of another SWC category
    function unusedReturn(int x, int y) public pure returns (int m, int n,int o) {
        m = y - x;
        o = m/2;
    }

    // x is not accessed 
    function neverAccessed(int test) public pure returns (int) {
        int z = 10;

        if (test > z) {
            return test - z;
        }

        return z;
    }

    function tupleAssignment(int p) public returns (int q, int r){
        (q, , r) = unusedReturn(p,2);

    }

}

unused_variables_fixed.yaml

description: Presence of unused variables
issues:
- id: SWC-131
  count: 0
  locations: []

SWC智能合约漏洞库

SWC-131/存在未使用的变量

CWE错误分类

整改方案

参考文献

示例合约