return complex(a,b)和return (a,b)有什么区别

return complex(a,b)和return (a,b)有什么区别
定义一个complex类后
complex complex::operator+(complex c2)
return complex(real+c2.real,imag+c2.imag)和return (real+c2.real,imag+c2.imag)为什么有区别?分别输出（6.5,12.5）和（12.5,0）