8.9.1 Determining When Procedures Require Explicit Interfaces

A procedure must have an explicit interface in the following cases:

• If the procedure has any of the following:
  
  
  • An optional dummy argument
    
    
  • A dummy argument that is an assumed-shape array, a pointer, or a target
    
    
  • A result that is array-valued or a pointer (functions only)
    
    
  • A result whose length is neither assumed nor a constant (character functions only)
    
    
  • A dummy argument that appears in a High Performance Fortran (HPF) data mapping directive
  
  
• If a reference to the procedure appears as follows:
  
  
  • With an argument keyword
    
    
  • As a reference by its generic name
    
    
  • As a defined assignment (subroutines only)
    
    
  • In an expression as a defined operator (functions only)
    
    
  • In a context that requires it to be pure
  
  
• If the procedure is elemental

For More Information:

• On optional arguments, see Section 8.8.1.
  
  
• On argument keywords in subroutine references, see Section 7.2.
  
  
• On argument keywords in function references, see Section 8.5.2.2.
  
  
• On user-defined generic procedures, see Section 8.9.3.
  
  
• On defined operators, see Section 8.9.4.
  
  
• On defined assignment, see Section 8.9.5.
  
  
• On array arguments, see Section 8.8.2.
  
  
• On pointer arguments, see Section 8.8.3.
  
  
• On pure procedures, see Section 8.5.1.2.
  
  
• On elemental procedures, see Section 8.5.1.3.
  
  
• On explicit interfaces when calling other languages, see your user manual or programmer's guide.