为什么我的向量中的字符串不更改值？

Question

I am trying to change the value of time in my vector of structs. The result should output 1430 as the value of shares[1].time though I get something different.

The shares is the name of my vector, I've already populated vector with values for each struct. Share is a struct which contains a string time.

//This is in the Share.cpp
vector<Share>shares  //vector of structs.

unsigned convTime = 1430;
std::stringstream out;

out << convTime;    //make convTime a string stream
shares[1].time = out.str();    


std::cout << shares[1].time;



struct Share
{
    //member all the information in the structure
    std::string date;
    std::string time;
    double price;
    double volume;
    double value;
    std::string condition; //not nessesary in this assignment
};

But my shares[1].time stays as its original value and not 1430. What could I be doing wrong?

Answer 1

You have to actually put objects in the vector before using them. When you write:

vector<Share> shares;

there are no objects in it yet. Going

shares[1].time

causes undefined behaviour. If you use the .at() function instead of [] then you get an exception instead of undefined behaviour.

To put objects in the vector, either use push_back() , or shares.resize(num_objects) .

Answer 2

Most likely, you have defined vector<Share> shares in Share.cpp, as you say, but also somewhere else !

This violates the One Definition Rule. Weird things may happen as a result. One of the things that definitely could happen is that shares[1] in one file isn't the same as shares[1] in another. You'd have two variables with the same name. But, remember that this is a could happen . Violating the One Definition Rule may break your program in all kinds of unexpected ways.