C++超詳細(xì)梳理lambda和function的使用方法

更新時(shí)間：2022年08月09日 16:07:07 作者：戀戀風(fēng)辰

C++在C11標(biāo)準(zhǔn)中引入了匿名函數(shù),即沒有名字的臨時(shí)函數(shù),又稱之為lambda表達(dá)式.lambda表達(dá)式實(shí)質(zhì)上是創(chuàng)建一個(gè)匿名函數(shù)/對(duì)象，這篇文章主要介紹了lambda和function的使用方法

lambda表達(dá)式

lambda表達(dá)式又稱為匿名表達(dá)式，是C11提出的新語法。[]存儲(chǔ)lambda表達(dá)式要捕獲的值，()內(nèi)的參數(shù)為形參，可供外部調(diào)用傳值。lambda表達(dá)式可以直接調(diào)用

 // 1  匿名調(diào)用
    [](string name)
    {
        cout << "this is anonymous" << endl;
        cout << "hello " << name << endl;
    }("zack");

上述代碼定義了一個(gè)匿名函數(shù)后直接調(diào)用。我們可以通過auto初始化一個(gè)變量存儲(chǔ)lambda表達(dá)式

 // 2 通過auto賦值
    auto fname = [](string name)
    {
        cout << "this is auto  " << endl;
        cout << "hello " << name << endl;
    };
    fname("Rolin");

通過auto定義fname，然后存儲(chǔ)了lambda表達(dá)式，之后調(diào)用fname即可。也可以通過函數(shù)指針的方式接受lambda表達(dá)式

    typedef void (*P_NameFunc)(string name);
    // 3 函數(shù)指針
    P_NameFunc fname2 = [](string name)
    {
        cout << "this is P_NameFunc " << endl;
        cout << "hello " << name << endl;
    };
    fname2("Vivo");

P_NameFunc定義了fname2函數(shù)指針接受了lambda表達(dá)式。也可以通過function對(duì)象接受lambda表達(dá)式，function類是C11新增的語法。

// 4 function
    function<void(string)> funcName;
    funcName = [](string name)
    {
        cout << "this is function " << endl;
        cout << "hello " << name << endl;
    };
    funcName("Uncle Wang");

用一個(gè)function對(duì)象接受了lambda表達(dá)式，同樣可以調(diào)用該function對(duì)象funcName達(dá)到調(diào)用lambda的效果。

談?wù)刲ambda的捕獲

1 值捕獲

    int age = 33;
    string name = "zack";
    int score = 100;
    string job = "softengineer";
    //值捕獲
    [age, name](string name_)
    {
        cout << "age is " << age << " name is " << name << " self-name is " << name_ << endl;
    }("Novia");

上述lambda表達(dá)式捕獲了age和name，是以值的方式來捕獲的。所以無法在lambda表達(dá)式內(nèi)部修改age和name的值，如果修改age和name，編譯器會(huì)報(bào)錯(cuò)，提示無法修改const常量，因?yàn)閍ge和name是以值的方式被捕獲的。

2 引用捕獲

    int age = 33;
    string name = "zack";
    int score = 100;
    string job = "softengineer";
    [&age, &name](string name_)
    {
        cout << "age is " << age << " name is " << name << " self-name is " << name_ << endl;
        name = "Xiao Li";
        age = 18;
    }("Novia");

[]里age和name前邊添加了&，此時(shí)age和name是以引用方式捕獲的。所以可以在lambda表達(dá)式中修改age和name的值。

C++的lambda表達(dá)式雖然可以捕獲局部變量的引用，達(dá)到類似閉包的效果，但不是真的閉包，golang和python等語言通過閉包捕獲局部變量后可以增加局部變量的聲明周期，C++無法做到這一點(diǎn)，所以下面的調(diào)用會(huì)出現(xiàn)崩潰。

vector<function<void(string)>> vec_Funcs;
void use_lambda2()
{
    int age = 33;
    string name = "zack";
    int score = 100;
    string job = "softengineer";
    vec_Funcs.push_back([age, name](string name_)
                        {   cout << "this is value catch " << endl;
                            cout << "age is " << age << " name is " << name << " self-name is " << name_ << endl; });
    //危險(xiǎn)，不要捕獲局部變量的引用
    vec_Funcs.push_back([&age, &name](string name_)
                        {   cout << "this is referenc catch" << endl;
                            cout << "age is " << age << " name is " << name << " self-name is " << name_ << endl; });
}
void use_lambda3()
{
    for (auto f : vec_Funcs)
    {
        f("zack");
    }
}
int main(){
    use_lambda2();
    use_lambda3();
}

use_lambda2中將lambda表達(dá)式存儲(chǔ)在function類型的vector里，當(dāng)use_lambda2結(jié)束后，里邊的局部變量都被釋放了，而vector中的lambda表達(dá)式還存儲(chǔ)著局部變量的引用，在調(diào)用use_lambda3時(shí)調(diào)用lambda表達(dá)式，此時(shí)訪問局部變量已經(jīng)被釋放了，所以導(dǎo)致程序崩潰。

3 全部用值捕獲，name用引用捕獲

    int age = 33;
    string name = "zack";
    int score = 100;
    string job = "softengineer";
    [=, &name]()
    {
        cout << "age is " << age << " name is " << name << " score is " << score << " job is " << job << endl;
        name = "Cui Hua";
    }();

通過=表示所有變量都以值的方式捕獲，如果希望某個(gè)變量以引用方式捕獲則單獨(dú)在這個(gè)變量前加&。

4 全部用引用捕獲，只有name用值捕獲

   int age = 33;
   string name = "zack";
   int score = 100;
   string job = "softengineer";
   [&, name]()
   {
        cout << "age is " << age << " name is " << name << " score is " << score << " job is " << job << endl;
   }();

通過&方式表示所有變量都已引用方式捕獲，如果希望某個(gè)變量以值方式捕獲則單獨(dú)在這個(gè)變量前加=。

萬能的function

我們可以用function存儲(chǔ)形參和返回值相同的一類函數(shù)指針，可調(diào)用對(duì)象，lambda表達(dá)式等。

void use_function()
{
    list<function<void(string)>> list_Funcs;
    //存儲(chǔ)函數(shù)對(duì)象
    list_Funcs.push_back(FuncObj());
    //存儲(chǔ)lambda表達(dá)式
    list_Funcs.push_back([](string str)
                         { cout << "this is lambda call " << str << endl; });
    //存儲(chǔ)全局函數(shù)
    list_Funcs.push_back(globalFun);
    for (const auto &f : list_Funcs)
    {
        f("hello zack");
    }
}

bind操作

C11同樣提供了bind操作，將原函數(shù)的幾個(gè)參數(shù)通過bind綁定傳值，返回一個(gè)新的可調(diào)用對(duì)象。

    //綁定全局函數(shù)
    auto newfun1 = bind(globalFun2, placeholders::_1, placeholders::_2, 98, "worker");
    //相當(dāng)于調(diào)用globalFun2("Lily",22, 98,"worker");
    newfun1("Lily", 22);
    //多傳參數(shù)沒有用，相當(dāng)于調(diào)用globalFun2("Lucy",28, 98,"worker");
    newfun1("Lucy", 28, 100, "doctor");
    auto newfun2 = bind(globalFun2, "zack", placeholders::_1, 100, placeholders::_2);
    //相當(dāng)于調(diào)用globalFun2("zack",33,100,"engineer");
    newfun2(33, "engineer");
    auto newfun3 = bind(globalFun2, "zack", placeholders::_2, 100, placeholders::_1);
    newfun3("coder", 33);

placeholders表示占位符，_1表示新生成函數(shù)的第一個(gè)參數(shù), _2表示新生成函數(shù)的第二個(gè)參數(shù)，將這些參數(shù)傳遞給原函數(shù)達(dá)到占位的效果，原函數(shù)的其余參數(shù)通過bind綁定固定值。

接下來定義類

class BindTestClass
{
public:
    BindTestClass(int num_, string name_) : num(num_), name(name_) {}
    static void StaticFun(const string &str, int age);
    void MemberFun(const string &job, int score);
public:
    int num;
    string name;
};

實(shí)現(xiàn)靜態(tài)函數(shù)和成員函數(shù)

void BindTestClass::StaticFun(const string &str, int age)
{
    cout << "this is static function" << endl;
    cout << "name is " << str << endl;
    cout << "age is " << age << endl;
}
void BindTestClass::MemberFun(const string &job, int score)
{
    cout << "this is member function" << endl;
    cout << "name is " << name << endl;
    cout << "age is " << num << endl;
    cout << "job is " << job << endl;
    cout << "score is " << score << endl;
}

我們通過bind綁定靜態(tài)成員函數(shù)

    //綁定類的靜態(tài)成員函數(shù),加不加&都可以
    // auto staticbind = bind(BindTestClass::StaticFun, placeholders::_1, 33);
    auto staticbind = bind(&BindTestClass::StaticFun, placeholders::_1, 33);
    staticbind("zack");

新生成的staticbind函數(shù)可以直接傳遞一個(gè)參數(shù)zack就完成了調(diào)用。接下來用bind綁定成員函數(shù)

    BindTestClass bindTestClass(33, "zack");
    // 綁定類的成員函數(shù),一定要傳遞對(duì)象給bind的第二個(gè)參數(shù)，可以是類對(duì)象，也可以是類對(duì)象的指針
    // 如果要修改類成員，必須傳遞類對(duì)象的指針
    auto memberbind = bind(BindTestClass::MemberFun, &bindTestClass, placeholders::_1, placeholders::_2);
    memberbind("coder", 100);
    auto memberbind2 = bind(BindTestClass::MemberFun, placeholders::_3, placeholders::_1, placeholders::_2);
    memberbind2("coder", 100, &bindTestClass);
    //綁定類成員時(shí)，對(duì)象必須取地址
    auto numbind = bind(&BindTestClass::num, placeholders::_1);
    std::cout << numbind(bindTestClass) << endl;

當(dāng)然也可以直接用function對(duì)象接受bind返回的結(jié)果

    // function接受bind返回的函數(shù)
    function<void(int, string)> funcbind = bind(globalFun2, "zack", placeholders::_1, 100, placeholders::_2);
    funcbind(33, "engineer");
    // function接受bind 成員函數(shù)
    function<void(string, int)> funcbind2 = bind(BindTestClass::MemberFun, &bindTestClass, placeholders::_1, placeholders::_2);
    funcbind2("docker", 100);
    function<void(string, int, BindTestClass *)> funcbind3 = bind(BindTestClass::MemberFun, placeholders::_3, placeholders::_1, placeholders::_2);
    funcbind3("driver", 100, &bindTestClass);
    // function 直接接受成員函數(shù),function的模板列表里第一個(gè)參數(shù)是類對(duì)象引用
    function<void(BindTestClass &, const string &, int)> functomem = BindTestClass::MemberFun;
    functomem(bindTestClass, "functomem", 88);
    // function 綁定類的靜態(tài)成員函數(shù)
    function<void(const string &)> funbindstatic = bind(&BindTestClass::StaticFun, placeholders::_1, 33);
    funbindstatic("Rolis");

lambda和bind的使用就介紹到這里

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