copy initialization and direct initialization

string s5("hiya");// direct initialization

string s6 = ("hiya");// copy  initialization

string s7 = "hiya";// copy initialization

string s8(10, 'c'); // direct initialization; s8 is cccccccccc

string s9 = string(10, 'c'); // copy initialization

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string s4(10, 'c');// s4 is cccccccccc

When we have a single initializer, we can use either the direct or copy form of
initialization. When we initialize a variable from more than one value, such as in the
initialization of s4 above, we must use the direct form of initialization:

Several values refers to the number of constructor arguments.

Consider...

string s4(10, 'c'); //s4 is "cccccccccc"

There are two (i.e. several) values - 10 and 'c'.

In that situation you can't get the same behaviour from e.g....

string s4 = 10, 'c';  // doesn't work as above

...because the value to assign is evaluated as an expression with a comma operator: the left hand 10 is discarded, then the expression evaluates as simply 'c', which becomes the only argument to the string constructor. There is no matching constructor taking a single char argument, nor anything the char can undergo Standard Conversion to, so you'll get a compilation error. (The closest constructor is...

basic_string( size_type count,
          CharT ch,
          const Allocator& alloc = Allocator() );

...for whatever that's worth....)

With C++11 you can use braces to create a single std::initializer_list argument with which you can call the constructor...

basic_string( std::initializer_list<CharT> init,
              const Allocator& alloc = Allocator() ); 

...but you could argue that's not calling a constructor with multiple values, but a single value containing sub-values....