Question

I am dealing with a lot of strings in my program. These string data don’t change through out the whole life time after they being read into my program.

But since the C++ string reserves capacity, they waste a lot of space that won’t be used for sure. I tried to release those spaces, but it didn’t work.

The following is the simple code that I tried:

string temp = '1234567890123456'; string str;  cout << str.capacity() << endl;     str.reserve(16);     cout << str.capacity() << endl;      // capacity is 31 on my computer      str += temp;     cout << str.capacity() << endl;      str.reserve(16);     cout << str.capacity() << endl;      // can't release. The capacity is still 31. 

(The compiler is Visual C++)

How could I release it?

Answer 1

When you call reserve, you’re making a request to change the capacity. Implementations will only guarantee that a number equal to or greater than this amount is reserved. Therefore, a request to shrink capacity may be safely ignored by a particular implementation.

However, I encourage you to consider whether this isn’t premature optimization. Are you sure that you’re really making so many strings that it’s a memory bottleneck for you? Are you sure that it’s actually memory that’s the bottleneck?

From the documentation for reserve:

This can expand or shrink the size of the storage space in the string, although notice that the resulting capacity after a call to this function is not necessarily equal to res_arg but can be either equal or greater than res_arg, therefore shrinking requests may or may not produce an actual reduction of the allocated space in a particular library implementation. In any case, it never trims the string content (for that purposes, see resize or clear, which modify the content).