Is ‘float a = 3.0;’ a correct statement?

It is not an error to declare float a = 3.0 : if you do, the compiler will convert the double literal 3.0 to a float for you.

However, you should use the float literals notation in specific scenarios.

1. For performance reasons:

   Specifically, consider:

   float foo(float x) { return x * 0.42; }
   

   Here the compiler will emit a conversion (that you will pay at runtime) for each returned value. To avoid it you should declare:

   float foo(float x) { return x * 0.42f; } // OK, no conversion required
   

2. To avoid bugs when comparing results:

   e.g. the following comparison fails :

   float x = 4.2;
   if (x == 4.2)
      std::cout << "oops"; // Not executed!
   

   We can fix it with the float literal notation :

   if (x == 4.2f)
      std::cout << "ok !"; // Executed!
   

   (Note: of course, this is not how you should compare float or double numbers for equality in general)

3. To call the correct overloaded function (for the same reason):

   Example:

   void foo(float f) { std::cout << "\nfloat"; }
   
   void foo(double d) { std::cout << "\ndouble"; }
   
   int main()
   {       
       foo(42.0);   // calls double overload
       foo(42.0f);  // calls float overload
       return 0;
   }
   

4. As noted by Cyber, in a type deduction context, it is necessary to help the compiler deduce a float :

   In case of auto :

   auto d = 3;      // int
   auto e = 3.0;    // double
   auto f = 3.0f;   // float
   

   And similarly, in case of template type deduction :

   void foo(float f) { std::cout << "\nfloat"; }
   
   void foo(double d) { std::cout << "\ndouble"; }
   
   template<typename T>
   void bar(T t)
   {
         foo(t);
   }
   
   int main()
   {   
       bar(42.0);   // Deduce double
       bar(42.0f);  // Deduce float
   
       return 0;
   }
   

Live demo