C++：使用向量和迭代器时出现错误：std::bad_alloc

Question

I am using mailio library and I get into trouble when I want to get all elements in a vector.

string msg = "Subject: " + mail -> subject() + "\nFrom: ";
for(auto address = ( mail -> from() ).addresses.begin(); address != ( mail -> from() ).addresses.end(); ++address ){ //address is not empty
    msg.append( address -> name );//     <----here
}

Here are the brief definations of class message :

class MAILIO_EXPORT message : public mime
{
...
public:
    mailboxes from() const;
...
}

struct MAILIO_EXPORT mailboxes
{
    ...
    std::vector<mail_address> addresses;
    ...
}

struct MAILIO_EXPORT mail_address
{
    ...
    std::string name;
    ...
}

The program terminates and returns get error: std::bad_alloc . I don't know what's wrong, though I still managed to solved it with following code:

string msg = "Subject: " + mail -> subject() + "\nFrom: ";
auto add = ( mail -> from() ).addresses; 
for(auto address = add.begin(); address != add.end(); ++address ){
     msg.append( address -> name );
}

Maybe I tried to modify one or some of these const values unconsciously. But I am not.

Answer 1

If the from() doesn't return a reference then it makes a copy of the struct, including copying the std::vector . This means in your loop you get two different vectors of which you take begin() and end() respectively. You're not allowed to loop between these since the begin iterator will most likely never match the end, especially before going out of bounds.

In the second code there's one copy and you use iterators from it so they will work.