C++ 在 if 语句设置条件为真和 or-ing 循环条件之间的行为差​​异

Question

我正在为我的 CS class 开发 Datalog 解释器，我 运行 遇到了一个 st运行ge 问题，我的规则评估需要太多遍才能完成。在查看我的代码后，我在下面做了两处修改，修复了我的评估以正确的遍数执行：

//original form
bool addedFacts = false;
for (X x: xs) {
    addedFacts = addedFacts || set.insert(x).second;
}
//modified form
bool addedFacts = false;
for (X x: xs) {
    if (set.insert(x).second) {
        addedFacts = true;
    }
}

对我来说，这两个代码结构在逻辑上是等价的。有没有一个执行正确，一个执行incorrectly/inefficiently的原因？ 这是发生的问题的可构建示例：

#include <iostream>
#include <set>
#include <vector>

using std::set;
using std::vector;
using std::cout;
using std::endl;

const int CAP = 100;

class Rule {
public:
    int factor;
    Rule(int factor) {
        this->factor = factor;
    }
    bool evaluateInefficient(set<int>& facts) {
        vector<int> data;
        bool addedFacts = false;
        for (int fact : facts) {
            data.push_back(fact);
        }
        for (int datum : data) {
            int newFact = datum * factor;
            if (newFact < CAP) {
                addedFacts = addedFacts || facts.insert(newFact).second;
            }
        }
        return addedFacts;
    }
    bool evaluate(set<int>& facts) {
        vector<int> data;
        bool addedFacts = false;
        for (int fact : facts) {
            data.push_back(fact);
        }
        for (int datum : data) {
            int newFact = datum * factor;
            if (newFact < CAP) {
                if (facts.insert(newFact).second) {
                    addedFacts = true;
                }
            }
        }
        return addedFacts;
    }
};

int doublyInefficient(vector<Rule>& rules) {
    set<int> facts;
    facts.insert(1);
    bool addedFacts = true;
    int passes = 0;
    while (addedFacts) {
        passes++;
        addedFacts = false;
        for (Rule rule : rules) {
            addedFacts = addedFacts || rule.evaluateInefficient(facts);
        }
    }
    return passes;
}

int singlyInefficient(vector<Rule>& rules) {
    set<int> facts;
    facts.insert(1);
    bool addedFacts = true;
    int passes = 0;
    while (addedFacts) {
        passes++;
        addedFacts = false;
        for (Rule rule : rules) {
            addedFacts = addedFacts || rule.evaluate(facts);
        }
    }
    return passes;
}

int efficient(vector<Rule>& rules) {
    set<int> facts;
    facts.insert(1);
    bool addedFacts = true;
    int passes = 0;
    while (addedFacts) {
        passes++;
        addedFacts = false;
        for (Rule rule : rules) {
            if (rule.evaluate(facts)) {
                addedFacts = true;
            }
        }
    }
    return passes;
}

int main(int argc, char* argv[]) {
    //build the rules
    vector<Rule> rules;
    rules.push_back(Rule(2));
    rules.push_back(Rule(3));
    rules.push_back(Rule(5));
    rules.push_back(Rule(7));
    rules.push_back(Rule(11));
    rules.push_back(Rule(13));
    //Show three different codes that should (in my mind) take the same amount of passes over the rules but don't
    cout << "Facts populated after " << doublyInefficient(rules) << " passes through the Rules." << endl;
    cout << "Facts populated after " << singlyInefficient(rules) << " passes through the Rules." << endl;
    cout << "Facts populated after " << efficient(rules) << " passes through the Rules." << endl;
    getchar();
}

我在 visual studio 2017 年 运行 处于调试和发布模式（32 位）时得到以下输出。据我所知，代码未优化。

Facts populated after 61 passes through the Rules.
Facts populated after 17 passes through the Rules.
Facts populated after 7 passes through the Rules.

Answer 1

addedFacts = addedFacts || set.insert(x).second;

和

if (set.insert(x).second) {
    addedFacts = true;
}

绝对不是一回事。第一块代码相当于：

if (!addedFacts) {
    addedFacts = set.insert(x).second;
}

!addedFacts 检查有很大不同。

Answer 2

由于短路评估而产生差异： 考虑 (expr1 || expr2) 形式的表达式。短路意味着如果 expr1 计算为 true，则表达式 expr2 根本不会被计算（参见 this online c++ standard draft，重点是我的）：

5.15 Logical OR operator

The || operator groups left-to-right. The operands are both contextually converted to bool (Clause [conv]). It returns true if either of its operands is true, and false otherwise. Unlike |, || guarantees left-to-right evaluation; moreover, the second operand is not evaluated if the first operand evaluates to true.

因此，在您的表达式 addedFacts || set.insert(x).second 中，从 addedFacts 第一次变为 true 的那一点起，表达式 set.insert(x).second 将不再执行。我想这是 "wrong" 行为，因为你的 set 将不包含相应的 xes。