我想写自己的String类，其接口类似于std :: string。字符串类不得使用动态内存分配。
我需要一个c-tor：
String(char* ptrToFirstCharInTab, char* ptrToLastElementInTab);
应该有一个包含不同（不知道）元素数量的选项卡，所以我在编译时不知道大小。
在我看来这是不可能的，因为如果在编译之前我们不知道数组的大小，那么我们就不能在没有动态分配的情况下创建它-当然，要为500个char创建缓冲区，然后String类只能是500个，这不是我的期望。
你有什么主意吗？也许有什么办法可以使我缩小以适应buffor？感谢您的帮助！
答案 0 :(得分：4)
您问：
你有什么主意吗？也许有什么办法可以使我缩到合适的程度呢？
理论上，可以。您可以使用预分配的缓冲区作为堆内存。但是，您必须编写自己的代码来管理该缓冲区。可行，但我不建议这样做。
答案 1 :(得分：1)
您问：
是否可以在不使用C ++中的堆的情况下创建String类？
实际上，是的，可以通过使用from PyQt5 import QtCore, QtGui, QtWidgets
class Foo(QtWidgets.QWidget):
def __init__(self, parent=None):
super(Foo, self).__init__(parent)
self.setGeometry(QtCore.QRect(200, 100, 800, 800))
self.button = Button()
self.paint = Createpaintwidget()
self.button.valuesChanged.connect(self.paint.set_size_squares)
self.button.valueChanged.connect(self.paint.set_size_round)
self.lay = QtWidgets.QVBoxLayout(self)
self.lay.addWidget(self.paint)
self.lay.addWidget(self.button)
class Createpaintwidget(QtWidgets.QWidget):
def __init__(self):
super().__init__()
self.sizeHint()
self.setBackgroundRole(QtGui.QPalette.Base)
self.setAutoFillBackground(True)
self._size = QtCore.QSizeF()
self._path = QtGui.QPainterPath()
self._rect = QtCore.QRectF()
self._type = QtGui.QRegion.Rectangle
self._factor = 1.0
self._pos = QtCore.QPointF()
self._initial_flag = False
fnt = self.font()
fnt.setPointSize(20)
self.setFont(fnt)
def showEvent(self, event):
if not self._initial_flag:
self._pos = self.rect().center()
self._initial_flag = True
@QtCore.pyqtSlot(int,int)
def set_size_squares(self, w, h):
self._size = QtCore.QSizeF(w, h)
self._type = QtGui.QRegion.Rectangle
self.updatePath()
@QtCore.pyqtSlot(int)
def set_size_round(self, v):
self._size = QtCore.QSizeF(v, v)
self._type = QtGui.QRegion.Ellipse
self.updatePath()
def paintEvent(self, event):
pen = QtGui.QPen()
brush = QtGui.QBrush(QtCore.Qt.black)
painter = QtGui.QPainter(self)
painter.setRenderHint(QtGui.QPainter.Antialiasing)
painter.setPen(pen)
painter.setBrush(brush)
painter.translate(self.rect().center())
painter.scale(self._factor, self._factor)
painter.translate(-self.rect().center())
painter.translate(self._pos)
painter.drawPath(self._path)
if self._type == QtGui.QRegion.Rectangle:
painter.fillRect(self._rect, QtGui.QBrush(QtCore.Qt.gray, QtCore.Qt.Dense7Pattern))
painter.setBrush(QtGui.QBrush(QtCore.Qt.NoBrush))
painter.drawRect(self._rect)
elif self._type == QtGui.QRegion.Ellipse:
painter.setBrush(QtGui.QBrush(QtCore.Qt.gray, QtCore.Qt.Dense7Pattern))
painter.drawEllipse(self._rect)
def mousePressEvent(self, event):
QtWidgets.QApplication.setOverrideCursor(QtGui.QCursor(QtCore.Qt.OpenHandCursor))
self._initial_pos = event.pos()
super().mousePressEvent(event)
def mouseMoveEvent(self, event):
delta = event.pos() - self._initial_pos
self._path.translate(delta)
self._rect.translate(delta)
self.update()
self._initial_pos = event.pos()
super().mouseMoveEvent(event)
def mouseReleaseEvent(self, event):
QtWidgets.QApplication.restoreOverrideCursor()
super().mouseReleaseEvent(event)
def updatePath(self):
fm = QtGui.QFontMetrics(self.font())
r = QtCore.QRectF(QtCore.QPointF(), self._size)
r.moveCenter(QtCore.QPointF())
self._rect = QtCore.QRectF(r)
self._path.moveTo(QtCore.QPointF())
p = QtCore.QPointF(self._size.width()/2 + 75 ,0)
self._path.lineTo(p)
self._path.lineTo(p + QtCore.QPoint(-16, -16))
self._path.lineTo(p)
self._path.moveTo(p)
self._path.lineTo(p + QtCore.QPoint(-16, 16))
self._path.lineTo(p)
self._path.addText(p + QtCore.QPoint(16, 0) , self.font(), "x")
self._path.moveTo(QtCore.QPointF())
p = QtCore.QPointF(0, -self._size.height()/2 - 75)
self._path.lineTo(p)
self._path.lineTo(p + QtCore.QPoint(16, 16))
self._path.lineTo(p)
self._path.moveTo(p)
self._path.lineTo(p + QtCore.QPoint(-16, 16))
self._path.addText(p + QtCore.QPoint(0, -16) , self.font(), "y")
if self._type == QtGui.QRegion.Rectangle:
pl = r.bottomLeft() + QtCore.QPointF(0, 75)
pr = r.bottomRight() + QtCore.QPointF(0, 75)
self._path.moveTo(pl)
self._path.lineTo(pr)
for p in (pl, pr):
self._path.moveTo(p+ QtCore.QPoint(0, -40))
self._path.lineTo(p+ QtCore.QPoint(0, 20))
self._path.moveTo(p+ QtCore.QPoint(10, -10))
self._path.lineTo(p+ QtCore.QPoint(-10, 10))
word = "{}".format(self._size.width())
p = QtCore.QPointF(r.center().x() - 0.5*fm.width(word) , r.bottom() + 100)
self._path.addText(p , self.font(), word)
pt = r.topLeft() + QtCore.QPointF(-75, 0)
pb = r.bottomLeft() + QtCore.QPointF(-75, 0)
self._path.moveTo(pt)
self._path.lineTo(pb)
for p in (pt, pb):
self._path.moveTo(p+ QtCore.QPoint(40, 0))
self._path.lineTo(p+ QtCore.QPoint(-20, 0))
self._path.moveTo(p+ QtCore.QPoint(10, -10))
self._path.lineTo(p+ QtCore.QPoint(-10, 10))
word = "{}".format(self._size.height())
p = QtCore.QPointF(r.left() -80 - fm.width(word) , r.center().y() + 0.5*fm.height())
self._path.addText(p , self.font(), word)
if self._type == QtGui.QRegion.Ellipse:
pl = r.bottomLeft() + QtCore.QPointF(0, 75)
pr = r.bottomRight() + QtCore.QPointF(0, 75)
self._path.moveTo(pl)
self._path.lineTo(pr)
for p in (pl, pr):
self._path.moveTo(p+ QtCore.QPoint(0, -self._size.height()/2 - 20))
self._path.lineTo(p+ QtCore.QPoint(0, 10))
self._path.moveTo(p+ QtCore.QPoint(10, -10))
self._path.lineTo(p+ QtCore.QPoint(-10, 10))
word = "{}".format(self._size.width())
p = QtCore.QPointF(r.center().x() - 0.5*fm.width(word) , r.bottom() + 100)
self._path.addText(p , self.font(), word)
self.update()
def wheelEvent(self, event):
self._factor *= 1.01**(event.angleDelta().y()/15.0)
self.update()
super().wheelEvent(event)
class Button(QtWidgets.QWidget):
valueChanged = QtCore.pyqtSignal(int)
valuesChanged = QtCore.pyqtSignal(int,int)
def __init__(self, parent=None):
super(Button, self).__init__(parent)
roundbutton = QtWidgets.QPushButton('Round')
squarebutton = QtWidgets.QPushButton('Square')
Alay = QtWidgets.QVBoxLayout(self)
Alay.addWidget(roundbutton)
Alay.addWidget(squarebutton)
self.value = QtWidgets.QLabel()
roundbutton.clicked.connect(self.getbuttonfunc)
squarebutton.clicked.connect(self.sqaurebuttonfunc)
@QtCore.pyqtSlot()
def getbuttonfunc(self):
number, ok = QtWidgets.QInputDialog.getInt(self, self.tr("Set Number"),
self.tr("Input:"), 1, 1)
if ok:
self.valueChanged.emit(number)
@QtCore.pyqtSlot()
def sqaurebuttonfunc(self):
number, ok = QtWidgets.QInputDialog.getInt(self, self.tr("Set Number"),
self.tr("Input:"), 1, 1)
if ok:
self.valuesChanged.emit(number, number)
if __name__ == '__main__':
import sys
app = QtWidgets.QApplication(sys.argv)
w = Foo()
w.show()
sys.exit(app.exec_())
或类似的平台相关功能来动态地在堆栈上分配内存。查看此其他答案以获取更多详细信息：
C++ How to allocate memory dynamically on stack?
我建议您反对，并绝对确保这是开始之前的最佳选择。
更新：
我使用_alloca创建了一个带有内联构造函数的示例，用于演示：
编译器资源管理器链接： https://godbolt.org/z/M1F5VD
完整代码：
gcc答案 2 :(得分：1)
我对您的问题有点困惑。您想要std:: string没有堆且没有大小限制。很抱歉为您带来这个：您不可能拥有无限的记忆力。
如果您有一个专用于字符串的内存池，而每个字符串的大小都不固定，则分配器可以这样做。 容器的默认分配器执行的是新的，但是您可以替换它而不必重复字符串的内部。