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Why don't std::vector's elements need a default constructor?


By : Yøgèsh Räghûwáñshí T
Date : October 16 2020, 11:12 PM
it helps some times std::vector doesn't need the default constructor because it never uses it. Every time it needs to construct an element, it does it by using the copy constructor, because every time it has something to copy: either existing vector element or an element you yourself supplied for copying through a method's parameter (explicitly or implicitly, by relying on a default argument)
You can write a class like that in exactly the same way: every time you need to construct a new element in your array, require the user to supply an element for copying. In this case constructing that original element becomes user's responsibility.
code :


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Safety of map operator[] when you dont want a default constructor


By : Jason Hansen
Date : March 29 2020, 07:55 AM
will be helpful for those in need I recently ran into an interesting bug with a program at work. I would like to know a little bit more about how operator[] works with maps. Consider the following example code: , from Visual Studio 2012 library
code :
mapped_type& operator[](key_type&& _Keyval)
    {   // find element matching _Keyval or insert with default mapped
    iterator _Where = this->lower_bound(_Keyval);
    if (_Where == this->end()
        || this->_Getcomp()(_Keyval, this->_Key(_Where._Mynode())))
        _Where = this->insert(_Where,
            pair<key_type, mapped_type>(
                _STD move(_Keyval),
                mapped_type()));  // <---- call to default constructor
    return (_Where->second);
    }

Dont allow use of default constructor in C++


By : Edgar Petrosyan
Date : March 29 2020, 07:55 AM
This might help you What is happening is a vexing parse. You're not declaring an object, but a function with name c and return type of class Point.
code :
Point p{newX, newY};
Point p{}; //default constructs

How to put elements of a list of vectors of different lengths into a data frame, with elements of vectors being separate


By : user1614499
Date : March 29 2020, 07:55 AM
Does that help The solution below (1) converts each list element into a dataframe where t() is used to get the element names to be column names in the dataframe, and (2) row-binds those dataframes together where data.table's fill argument is helpful for the inconsistent vector lengths/names.
code :
mylist <- list(
    one = c(SW=0.49, GHS=0.46, GS=0.03),
    two = c(GHS=0.07, GW = 0.03, LLW=0.03, GS=0.00),
    six = c(GHS=0.41, SW=0.42, TD=0.09)
)

temp <- lapply(mylist, function(x) data.frame(t(x)))
data.table::rbindlist(temp, fill=TRUE)
     SW  GHS   GS   GW  LLW   TD
1: 0.49 0.46 0.03   NA   NA   NA
2:   NA 0.07 0.00 0.03 0.03   NA
3: 0.42 0.41   NA   NA   NA 0.09

How do C++ Vectors not overwrite within memory space with default constructor?


By : user3418424
Date : March 29 2020, 07:55 AM
it fixes the issue This is more of a theoretical question than a problem. , How are these answers?

Using custom class with vectors: 'std::vector' default constructor error


By : SDXT
Date : March 29 2020, 07:55 AM
With these it helps You need to have a default constructor to use std::vector. The default constructor is the one that has no arguments, i.e., Item::Item() { ... }
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