【发布时间】：2017-01-02 22:53:14
【问题描述】：
我的 C++ 代码中不断出现分段错误，我通过执行线程使用 GDB 进行了跟踪，但看不到问题出在哪里。我的程序从主类 AsTheCrowFlies 中的 Vertex 对象列表创建一个 Graph 对象。 Graph 对象的实际创建发生在 makeGraph(const std::vector) 中。 makeGraph 中问题开始的那一行是我调用另一个函数 checkEdges(Map map, const Vertex &To, const &From) 的地方，如下所示：
bool Graph::checkEdges(Map &map, const Vertex &To, const Vertex &From) {
if(map.find(To)!=map.end()) {
std::vector<Edge> edges = map[To];
for (auto itr = edges.begin(); itr!=edges.end(); ++itr) {
if(itr->getDestination()==From)
return true;
}
}
return false;
}
根据 GDB 的输出，当 for 循环中的 if 语句执行并尝试使用 Vertex 类中的重载 == 函数时：
bool Vertex::operator == (const Vertex &other) const {
if (name==other.name && latitude==other.latitude &&
longitude == other.longitude) {
return true;
}
return false;
}
它给出了错误消息：“Cannot access memory at address 0x4a8>” 当我在 GDB 中使用 bt 命令时，错误消息的完整列表是：
Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b8f6f3 in std::string::size() const ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
(gdb) bt
#0 0x00007ffff7b8f6f3 in std::string::size() const ()
from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#1 0x000000000040a729 in std::operator==<char> (__lhs=" ",
__rhs=<error reading variable: Cannot access memory at address 0x4a8>)
at /usr/include/c++/4.8/bits/basic_string.h:2495
#2 0x000000000041a30d in Vertex::operator== (this=0x671b58, other=...)
at Vertex.cpp:50
#3 0x0000000000415fb7 in Graph::checkEdges (this=0x7fffffffe880,
map=std::unordered_map with 50 elements = {...}, To=..., From=...)
at Graph.cpp:364
#4 0x00000000004157cf in Graph::makeGraph (this=0x7fffffffe880,
input=std::vector of length 50, capacity 50 = {...}) at Graph.cpp:161
#5 0x0000000000414bea in Graph::setVertices (this=0x7fffffffe880,
v=std::vector of length 50, capacity 64 = {...}) at Graph.cpp:38
#6 0x0000000000402b62 in AsTheCrowFlies::menu (this=0x7fffffffe800,
filename=0x7fffffffec48 "cap_cities.txt") at AsTheCrowFlies.cpp:17
#7 0x000000000041a05c in main (argc=2, argv=0x7fffffffe9c8) at main.cpp:24
这里是完整版的makeGraph(const Vertex input)：
Map Graph::makeGraph(std::vector<Vertex> input) {
Map new_graph;
size_t size, n, mid,index;
size = input.size();
n = size - 1;
mid = ((size + size%2)/2)-1;
if (size<1)
return new_graph;
double mean,max,min,median; //double variables for mean, median, maximum, minimum, sum of squares and standard deviation created
//vector used to hold the distances between adjacent vertices
std::vector<double> dist(size);
for (int i = 0; i < size; i++) {
mean=0.0; //set average/mean to "0"
if(new_graph.find(input[i])==new_graph.end()) { //if the current Vertex has no map entry, add it here
std::vector<Edge> edges(size);
new_graph[input[i]] = edges;
}
//Create and sort duplicate list with each Vertex moved to
the front in turn
std:vector<Vertex> list(size);
list = input;
sort(list.begin(),list.end(),[&](const Vertex &v1, const Vertex &v2){return findMinDist(input[i].getLatitude(),input[i].getLongitude(),v1.getLatitude(),v1.getLongitude()) < findMinDist(input[i].getLatitude(),input[i].getLongitude(),v2.getLatitude(),v2.getLongitude()) ;});
//Gets mean and fills distance tracking vector
for (int j = 0; j < size; j++) {
dist[j]=findMinDist(input[i].getLatitude(),input[i].getLongitude(),input[j].getLatitude(),input[j].getLongitude());
mean+=dist[j]/size;//accumulates mean value
}
min = dist[1];
max = dist[n];
if(size%2==0) { //calculates median distance value for...
median = (dist[mid]+dist[mid+1])/2; //...even sized vectors and...
} else {median = dist[mid];} //...odd sized vectors
if(mean >= median) { //index value is calucated for...
index = (mid+1)*(mean-min)/(max-min); //...case where mean is greater than or equal to median and...
} else {index = (mid+1)*(median-min)/(max-min);} //...case where mean is less than median and...
for (int k = 0; k < index; k++) { //
if(!checkEdges(new_graph,list[0],list[k])){
new_graph[list[0]].push_back(Edge(list[0],list[k],dist[k]));
}
if(k!=0) {
if(new_graph.find(list[k])==new_graph.end()) {//SEGMENTATION
//FAULT STARTS
//HERE
std::vector<Edge> edges(size);
new_graph[list[k]]=edges;
new_graph[list[k]].push_back(Edge(list[k],list[0],dist[k]));
} else {
if(!checkEdges(new_graph,list[k],list[0])) {
new_graph[list[k]].push_back(Edge(list[k],list[0],dist[k]));
}
}
}
}
dist.clear();
}
return new_graph;
}
getDestination 的签名是
Vertex& Edge::getDestination() {return destination;}
我看着这个，不明白为什么这里会出现分段错误。请帮忙！我已经包含了似乎发生错误的代码的相关部分，但如果您需要更多代码来帮助了解发生了什么，请告诉我。
【问题讨论】：
-
您是否检查过
From是否有效？如果是nullptr就可以解释很多了。 -
您是否尝试过使用正确的类型而不是 auto ？自动有时表现得很奇怪
-
这可能会更快：
std::vector<Edge> edges = map[To];=>const std::vector<Edge> &edges = map[To]; -
你能给我们足够的代码来复制这个问题吗？
-
getDesintation的签名是什么？From来自哪里？