C++ 错误：取临时地址 [-fpermissive]
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Error: taking address of temporary [-fpermissive]
提问by Hyman Riales
I've been looking into this for a few hours, to no avail. Basically I have
我已经研究了几个小时，但无济于事。基本上我有
struct rectangle {
int x, y, w, h;
};
rectangle player::RegionCoordinates() // Region Coord
{
rectangle temp;
temp.x = colRegion.x + coordinates.x;
temp.w = colRegion.w;
temp.y = colRegion.y + coordinates.y;
temp.h = colRegion.h;
return temp;
}
// Collision detect function
bool IsCollision (rectangle * r1, rectangle * r2)
{
if (r1->x < r2->x + r2->w &&
r1->x + r1->w > r2->x &&
r1->y < r2->y + r2->h &&
r1->y + r1->h > r2->y)
{
return true;
}
return false;
}
//blah blah main while loop
if (IsCollision(&player1.RegionCoordinates(), &stick1.RegionCoordinates())) //ERROR
{
player1.score+=10;
stick1.x = rand() % 600+1;
stick1.y = rand() % 400+1;
play_sample(pickup,128,128,1000,false);
}
Any ideas? I'm sure it's something really obvious but for the life of me I can't figure it out.
有任何想法吗？我敢肯定这是非常明显的事情，但对于我的生活，我无法弄清楚。
回答by Angew is no longer proud of SO
RegionCoordinates()returns an object by value. This means a call to RegionCoordinates()returns a temporary instance of rectangle. As the error says, you're trying to take the address of this temporary object, which is not legal in C++.
RegionCoordinates()按值返回一个对象。这意味着调用RegionCoordinates()返回 的临时实例rectangle。正如错误所说，您正在尝试获取此临时对象的地址，这在 C++ 中是不合法的。
Why does IsCollision()take pointers anyway? It would be more natural to take its parameters by const reference:
为什么IsCollision()无论如何都要接受指针？通过 const 引用获取其参数会更自然：
bool IsCollision (const rectangle &r1, const rectangle &r2) {
if (r1.x < r2.x + r2.w &&
r1.x + r1.w > r2.x &&
r1.y < r2.y + r2.h &&
r1.y + r1.h > r2.y) {
return true;
}
return false;
}
//blah blah main while loop
if (IsCollision(player1.RegionCoordinates(), stick1.RegionCoordinates())) //no error any more
{
player1.score+=10;
stick1.x = rand() % 600+1;
stick1.y = rand() % 400+1;
play_sample(pickup,128,128,1000,false);
}
回答by Shafik Yaghmour
Since IsCollisiontakes a rectangle *and you are taking the address of the result here:
由于IsCollision需要 arectangle *并且您在此处获取结果的地址：
if (IsCollision(&player1.RegionCoordinates(), &stick1.RegionCoordinates()))
You most likely are returning a rectangleback from RegionCoordinates()which is a temporary variable since it will disappear after the ifstatement is done. If you assign the result of RegionCoordinates()to a variable then it will no longer be a temporary and you can then take the address of it:
你最有可能是返回一个rectangle回从RegionCoordinates()它是一个临时变量，因为以后会消失if语句完成。如果您将 的结果分配RegionCoordinates()给一个变量，那么它将不再是临时的，您可以获取它的地址：
rectangle r1 = player1.RegionCoordinates() ;
rectangle r2 = stick1.RegionCoordinates() ;
if (IsCollision(&r1, &r2))
Alternatively you could take the parameters as constreferences which would be the more C++ way of doing it:
或者，您可以将参数作为const参考，这将是更多的 C++ 方式：
bool IsCollision (const rectangle &r1, const rectangle &r2)
回答by Andy Prowl
Given the kind of error you are getting, I must assume RegionCoordinates()is returning an object by value, thus causing the creation of a temporary, and you are taking the address of that temporary.
鉴于您遇到的错误类型，我必须假设RegionCoordinates()是按值返回一个对象，从而导致创建一个临时，并且您正在获取该临时的地址。
The address-of operator requires an lvalueas its operand, but you are applying it to an rvalue(temporaries are rvalues).
address-of 运算符需要一个左值作为其操作数，但您将其应用于右值（临时值是rvalues）。
You could do this (if you are not using C++11, replace autowith the type returned by RegionCoordinates):
您可以这样做（如果您不使用 C++11，请替换auto为 返回的类型RegionCoordinates）：
auto rcPlayer1 = player1.RegionCoordinates();
auto rcStick1 = player1.RegionCoordinates();
if (IsCollision(&rcPlayer1, &rcStick1)) //ERROR
{
player1.score+=10;
stick1.x = rand() % 600+1;
stick1.y = rand() % 400+1;
play_sample(pickup,128,128,1000,false);
}
Alternatively, you can change IsCollisionso that it accepts referencesrather than pointers, as suggested by Angew in his answer.
或者，您可以进行更改IsCollision，使其接受引用而不是指针，正如 Angew 在他的回答中所建议的那样。
