int Tcl_ExprLongObj(interp, objPtr, longPtr)
int Tcl_ExprDoubleObj(interp, objPtr, doublePtr)
int Tcl_ExprBooleanObj(interp, objPtr, booleanPtr)
int Tcl_ExprObj(interp, objPtr, resultPtrPtr)
These four procedures all evaluate an expression, returning the result in one of four different forms. The expression is given by the objPtr argument, and it can have any of the forms accepted by the expr command.
The interp argument refers to an interpreter used to evaluate the expression (e.g. for variables and nested Tcl commands) and to return error information.
For all of these procedures the return value is a standard Tcl result: TCL_OK means the expression was successfully evaluated, and TCL_ERROR means that an error occurred while evaluating the expression. If TCL_ERROR is returned, then a message describing the error can be retrieved using Tcl_GetObjResult. If an error occurs while executing a Tcl command embedded in the expression then that error will be returned.
If the expression is successfully evaluated, then its value is returned in one of four forms, depending on which procedure is invoked. Tcl_ExprLongObj stores an integer value at *longPtr. If the expression's actual value is a floating-point number, then it is truncated to an integer. If the expression's actual value is a non-numeric string then an error is returned.
Tcl_ExprDoubleObj stores a floating-point value at *doublePtr. If the expression's actual value is an integer, it is converted to floating-point. If the expression's actual value is a non-numeric string then an error is returned.
Tcl_ExprBooleanObj stores a 0/1 integer value at *booleanPtr. If the expression's actual value is an integer or floating-point number, then they store 0 at *booleanPtr if the value was zero and 1 otherwise. If the expression's actual value is a non-numeric string then it must be one of the values accepted by Tcl_GetBoolean such as “yes” or “no” or else an error occurs.
If Tcl_ExprObj successfully evaluates the expression, it stores a pointer to the Tcl object containing the expression's value at *resultPtrPtr. In this case, the caller is responsible for calling Tcl_DecrRefCount to decrement the object's reference count when it is finished with the object.