Which of the following are correct using the constructor? A. string s1; B. string s2 (s1); C. string s3 {7}; D. string s4 {‘a’}; E. string s5{7,‘a’}; Choose the most appropriate answer from the options given below:
A, B, D only
In C++11, brace initialisation of std::string prefers the initializer_list<char> constructor whenever the braced values can be converted to char, while parentheses select the ordinary constructors. So string s(7,'a') and string s{7,'a'} mean different things.
A — string s1; calls the default constructor and creates an empty string. Correct.
B — string s2 (s1); calls the copy constructor and makes s2 a copy of s1. Correct.
C — string s3 {7}; There is no string constructor that takes a length alone; the braces turn 7 into a single character with code 7 (a control character), not a string of length 7. This is a misuse of the constructor.
D — string s4 {‘a’}; uses the initializer-list constructor and creates the one-character string "a". Correct.
E — string s5{7,‘a’}; looks like the fill constructor, but with braces it builds the two-character string made of code 7 and 'a', not seven copies of 'a'; the fill constructor needs parentheses, string s5(7,'a');.
Combinations containing C or E include a misused constructor, and the pair A and B alone leaves out the valid declaration D.
A, B, D only option is correct.
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