內部移動的std :: vector元素和QVector

Question

有兩個向量std :: vector和QVector 。 我們必須檢查元素在插入時如何“移動”。 （用五個元素構造了兩個向量，並插入了零個元素）我有以下代碼：

#include <QVector>
#include <QTextStream>

struct MoveTest
{
    int i;

    MoveTest()                       {}
    MoveTest(const MoveTest& other)  {QTextStream(stdout) << "constr copy" << endl;}
    MoveTest(MoveTest &&other)       {QTextStream(stdout) << "constr move" << endl;}
    ~MoveTest()                      {}

    inline MoveTest&    operator=   (const MoveTest& other) {QTextStream(stdout) << "copy" << endl;}
    inline MoveTest&    operator=   (MoveTest &&other)      {QTextStream(stdout) << "move" << endl;}
};

int main(int argc, char *argv[])
{
    QTextStream(stdout) << "std::move:" << endl;
    MoveTest t1;
    MoveTest t2(std::move(t1));
    t1 = std::move(t2);

    QTextStream(stdout) << "QVector:" << endl;
    QVector<MoveTest> qmTest(5);
    qmTest.insert(qmTest.begin(), MoveTest());

    QTextStream(stdout) << "std::vector:" << endl;
    std::vector<MoveTest> mTest(5);
    mTest.insert(mTest.begin(), MoveTest());

    return 0;
}

我的輸出是gcc 4.7.2，QMAKE_CXXFLAGS + = -std = c ++ 0x：

std::move:
constr move
move
QVector:
constr copy
constr copy
constr copy
constr copy
constr copy
constr copy
copy
copy
copy
copy
copy
copy
std::vector:
constr move
constr copy
constr copy
constr copy
constr copy
constr copy

如何插入具有內部移位的元素而不進行復制？ 需要哪些GCC標志？

Answer 1

由於您的move運算符可以引發異常，因此std::vector不能使用它。 如果操作員在調整大小過程中途拋出異常，該怎么辦？ 如果它不能引發異常並且vector實現可以使用它，則將其聲明為noexcept 。

Answer 2

可能對某人有用。

struct MoveTest
{
    int i;

    MoveTest()                      {}
    MoveTest(MoveTest&&) noexcept   {std::cout << "constr move\n";}
    MoveTest(const MoveTest&)       {std::cout << "constr copy\n";}
    ~MoveTest() noexcept            {}

    MoveTest&   operator=   (MoveTest&&) noexcept   {std::cout << "move\n"; return *this;}
    MoveTest&   operator=   (const MoveTest&)       {std::cout << "copy\n"; return *this;}
};
Q_DECLARE_TYPEINFO(MoveTest, Q_MOVABLE_TYPE);

int main(int argc, char *argv[])
{
    std::cout << "std::move:\n";
    MoveTest t1;
    MoveTest t2(std::move(t1));
    MoveTest t3(std::move_if_noexcept(t2));
    t2 = std::move(t3);
    t1 = std::move_if_noexcept(t2);
    std::cout << "\n";

    std::cout << "QVector:\n";
    QVector<MoveTest> qmTest(5);
    qmTest.insert(qmTest.begin(), MoveTest());
    std::cout << "\n";

    std::cout << "std::vector:\n";
    std::vector<MoveTest> mTest(5);
    mTest.insert(mTest.begin(), MoveTest());

    return 0;
}

出：

std::move:
constr move
constr move
move
move

QVector:
constr copy
constr copy

std::vector:
constr move
constr move
constr move
constr move
constr move
constr move

Answer 3

QT中的容器肯定能夠應對移動語義。 運行以下示例，自己看看。

#include <QCoreApplication>
#include <QVector>
#include <iostream>

struct MoveTest
{
    int i;

    MoveTest()                      {}
    MoveTest(MoveTest&&) noexcept   {std::cout << "constr move\n";}
    MoveTest(const MoveTest&)       {std::cout << "constr copy\n";}
    ~MoveTest() noexcept            {}

    MoveTest&   operator=   (MoveTest&&) noexcept   {std::cout << "move\n"; return *this;}
    MoveTest&   operator=   (const MoveTest&)       {std::cout << "copy\n"; return *this;}
};
Q_DECLARE_TYPEINFO(MoveTest, Q_MOVABLE_TYPE);

int main(int argc, char *argv[])
{
    std::cout << "std::move:\n";
    MoveTest t1;
    MoveTest t2(std::move(t1));
    MoveTest t3(std::move_if_noexcept(t2));
    t2 = std::move(t3);
    t1 = std::move_if_noexcept(t2);
    std::cout << "\n";

    std::cout << "QVector:\n";
    QVector<MoveTest> qmTest;
    int i=5;
    while(i) {
        qmTest.append(MoveTest());
        --i;
    }
    std::cout << "\n";

    std::cout << "std::vector:\n";
    std::vector<MoveTest> mTest(5);
    mTest.insert(mTest.begin(), MoveTest());

    return 0;
}

由於兩個對象具有相似的用法，並不意味着所有成員函數都相同。 檢查下面的輸出。 出：

std::move:
constr move
constr move
move
move

QVector:
constr move
constr move
constr move
constr move
constr move

std::vector:
constr move
constr move
constr move
constr move
constr move
constr move
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