7

有两个向量std :: vector和QVector。我们必须检查插入时如何“移位”元素。（用五个元素构造两个向量并插入零元素）我有这段代码：

#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 gcc 4.7.2 输出，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

如何在不复制的情况下插入具有内部移位的元素？需要哪些 GCC 标志？

4

3 回答 3

5

由于您的移动运算符可以抛出异常，std::vector因此不能使用它。如果操作员在调整大小的过程中途抛出异常会怎样？如果它不能抛出异常并且向量实现可以使用它，则将其声明为noexcept 。

于 2012-12-27T10:46:25.110 回答
1

可能对某人有用。

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;
}

出去：

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
于 2012-12-28T08:55:56.287 回答
1

QT 中的容器肯定能够处理移动语义。运行下面的示例并亲自查看。

#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;
}

由于两个对象具有相似的用途，这并不意味着所有的成员函数都是相同的。检查下面的输出。出去：

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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于 2017-10-20T13:22:27.387 回答