Error using .reserve() to reserve space for a vector

Question

I am attempting to reserve memory for a med_array vector as seen below

vector <int> med_array = {};
med_array.reserve(50000);
MedianFinder();
void addNum(int num);
double findMedian();
vector<int> get_array();
void print_array();

However, I get an error stating:

medianfinderheader.h:10:4: error: 'med_array' does not name a type
   10 |    med_array.reserve(50000);

I have no idea why this is happening.

Answer 1

I figured it out.reserve needs to be run inside a function I can't define in the class header like I was trying to do.

Answer 2

Taking into account the error message, these lines:

vector <int> med_array = {};
med_array.reserve(50000);

are in a (global) namespace where you may place only declarations. But this line:

med_array.reserve(50000);

is not a declaration. So the compiler issues the error. That is, the compiler tries to interpret the line as a declaration, but the name med_array is not a type specifier.

From the C++ 20 Standard (9.8 Namespaces)

1 A namespace is an optionally-named declarative region . The name of a namespace can be used to access entities declared in that namespace; that is, the members of the namespace. Unlike other declarative regions, the definition of a namespace can be split over several parts of one or more translation units.

Pay attention to that this declaration:

vector <int> med_array = {};

can be written simpler, like:

vector <int> med_array;

Or, you could initially declare the vector with 50000 elements, like:

vector <int> med_array( 50'000 );