C++ 分段错误访问类数据成员而不显式使用指针

Question

Note: Similar questions have been asked, but nearly all that I've found seem to make direct use of a pointer which may not be relevant here

For this snippet of code

#include <iostream>
#include <string>
#include <vector>
#include <map>
using namespace std;
class Unit{
public:
    string name;
    char value;
    vector<Unit> inner;
    Unit(string Name){
        name=Name;
    }
    Unit(char Value){
        name="Character";
    }
};
    class Instruction{
    public:
        int size;
        string type;
        string value;
        map<string, vector<Unit>> vars;
        vector<Instruction> inner;
        vector<Instruction> outer;
        Instruction(string Type,string Value,vector<Instruction>& Outer,map<string, vector<Unit>> &Vars){
            type=Type;
            value=Value;
            outer=Outer;
            vars=Vars;
            size=0;
        }
        Instruction(string Type,vector<Instruction>& Outer,map<string, vector<Unit>> &Vars){
            type=Type;
            outer=Outer;
            vars=Vars;
            size=0;
        }
        bool matches(vector<Unit> &info,int position=0, int vectLocation=0){
            cout<<value<<'\n'; //Segmentation Fault Occurs Here
            return false;
        }
    };
void breakUp(vector<Unit>&u,string s){
    for(int i=0;i<s.size();i++){
        u.push_back(Unit(s[i]));
    }
}


int main(){
    //String to be matched
    vector<Unit> v;
    breakUp(v,"For the love of tests");
    //Instruction Vector
    vector<Instruction> i;
    //Var Vector
    map<string, vector<Unit>> x;
    //Add initial instruction
    Instruction constant("Constant","",i,x);
    constant.inner.push_back(Instruction("String","For the love of tests",constant.inner,x));

    //Test match
    bool result=i[0].matches(v);
    if(result==true){
        cout<<"Match True\n";
    }else{
        cout<<"Match False\n";
    }

    return 0;
}

I get a segmentation fault when attempting to access one of the data members within the matches function. This also occurs when it attempts to print size or type . I have not been able to discern the exact cause of this memory issue and would appriciate any advice or considerations.

Answer 1

The reason for SIGSEGV seems to be

bool result=i[0].matches(v);

When you did

vector<Instruction> i;

A new std::vector object was created with size 0.

Now, when you try to access the first element by the above statement, it may give Segmentation fault.

Suggestion:

1) Use const std::string & instead of std::string in the constructor. Using std::string will lead to the string getting copied twice (which won't be good for performance).

Answer 2

vector<Instruction> i; ... bool result=i[0].matches(v);

i is empty, so accessing i[0] yields undefined behavior.