问题
I have an Objective-C++ class that I wish to have a property which is a C++ std::vector<std::string>. I've declared it like this:
@interface WordModel : NSObject
@property std::vector<std::string> words;
@end
When I attempt to add words to the vector nothing happens. For example, in the -init method, I'm doing this:
std::string nextLine;
while(wordsFile.good())
{
std::getline(wordsFile, nextLine);
if (nextLine.length() > 0)
{
self.words.push_back(nextLine);
}
}
After calling push_back() the debugger reports the self.words vector as still being empty. Why? There are no error messages either during compilation or at runtime.
If I change the property into an ivar, everything works as expected. That is, if I make it into this:
@interface WordSetModel : NSObject
{
std::vector<std::string> words;
}
@end
I'm confused as to why there's such a difference.
回答1:
An Objective-C property is simply a pair of getter and setter methods that you can access using a dot notation. For a property @property std::vector<std::string> words;, the getter and setter methods would be:
- (std::vector<std::string>)words;
- (void)setWords:(std::vector<std::string>)w;
(I am assuming that you didn't implement the getters and setters, so it auto-synthesized them. But even if you implemented them yourself, I believe it would still have to have the above type signatures.)
When you read from self.words, that is just syntactic sugar for a call to the getter: [self words]. Notice that the getter returns an std::vector by value. I think that that is your problem here. When you pass or return something by value, it is copied. So you are getting a temporary copy of the vector, pushing to it, and then discarding it. The vector in the underlying instance variable is unchanged. For a method to be able to give you something that can allow you to change the underlying vector, it would have to return to you a C++ "reference", something like std::vector<std::string> &. I am not sure whether you can do that with an Objective-C property (I haven't tried).
来源：https://stackoverflow.com/questions/51979119/using-a-stdvector-as-an-objective-c-property