C++“访问冲突读取位置”错误
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C++ "Access violation reading location" Error
提问by Alex Alex
I have the following Vertexstruct in a Graphclass:
我Vertex在一个Graph类中有以下结构：
struct Vertex
{
string country;
string city;
double lon;
double lat;
vector<edge> *adj;
Vertex(string country, string city, double lon, double lat)
{
this->country = country;
this->city = city;
this->lon = lon;
this->lat = lat;
this->adj = new vector<edge>();
}
};
When calling the method I've written called getCost(), I keep getting the same Unhandled Exception
调用我编写的方法时getCost()，我不断收到相同的未处理异常
Access violation reading location 0x00000048
访问冲突读取位置 0x00000048
and I cannot figure out why.
我不知道为什么。
the getCost()method:
的getCost()方法：
void Graph::getCost(string from, string to)
{
Vertex *f = (findvertex(from));
vector<edge> *v = f->adj; // Here is where it gives the error
vector<edge>::iterator itr = v->begin();
for (; itr != v->end(); itr++)
{
if (((*itr).dest)->city == to)
cout << "\nCost:-" << (*itr).cost;
}
}
The method findvertex()returns a value of type Vertex*. Why do I keep receiving this error?
该方法findvertex()返回一个类型的值Vertex*。为什么我一直收到这个错误？
the findVertex method:
findVertex 方法：
Vertex* Graph::findvertex(string s)
{
vmap::iterator itr = map1.begin();
while (itr != map1.end())
{
if (itr->first == s){
return itr->second;
}
itr++;
}
return NULL;
}
Where map1is defined:
在哪里map1定义：
typedef map< string, Vertex *, less<string> > vmap;
vmap map1;
回答by
You haven't posted the findvertexmethod, but Access Reading Violation with an offset like 0x00000048means that the Vertex* f;in your getCost function is receiving null, and when trying to access the member adjin the nullVertex pointer (that is, in f), it is offsetting to adj(in this case, 72 bytes ( 0x48 bytes in decimal )), it's reading near the 0or nullmemory address.
您还没有贴findvertex的方法，但使用读冲突与像偏移0x00000048方式的Vertex* f;在你的getCost函数接收空，而当试图访问该成员adj的null顶点指针（即在f），它被抵消到adj（在这种情况下，72 字节（十进制 0x48 字节）），它在0或null内存地址附近读取。
Doing a read like this violates Operating-System protected memory, and more importantly means whatever you're pointing at isn't a valid pointer. Make sure findvertexisn't returning null, or do a comparisong for null on fbefore using it to keep yourself sane (or use an assert):
像这样读取会违反操作系统保护的内存，更重要的是意味着您指向的任何内容都不是有效指针。确保findvertex没有返回 null，或者f在使用它之前对 null 进行比较以保持理智（或使用断言）：
assert( f != null ); // A good sanity check
assert( f != null ); // A good sanity check
EDIT:
编辑：
If you have a mapfor doing something like a find, you can just use the map's findmethod to make sure the vertex exists:
如果你有一个map像查找这样的东西，你可以使用地图的find方法来确保顶点存在：
Vertex* Graph::findvertex(string s)
{
vmap::iterator itr = map1.find( s );
if ( itr == map1.end() )
{
return NULL;
}
return itr->second;
}
Just make sure you're still careful to handle the error case where it does return NULL. Otherwise, you'll keep getting this access violation.
只要确保您仍然小心处理它返回的错误情况NULL。否则，您将不断收到此访问冲突。
回答by Alex Alex
Vertex *f=(findvertex(from));
if(!f) {
cerr << "vertex not found" << endl;
exit(1) // or return;
}
Because findVertexcan return NULLif it can't find the vertex.
因为如果找不到顶点findVertex可以返回NULL。
Otherwise this f->adj;is trying to do
否则这f->adj;是试图做
NULL->adj;
Which causes access violation.
这会导致访问冲突。
