Item 32: Postpone variable definitions as long as possible.
So you subscribe to the C philosophy that variables should be defined at the beginning of a block. Cancel that subscription! In C++, it's unnecessary, unnatural, and expensive.Remember that when you define a variable of a type with a constructor or destructor, you incur the cost of construction when control reaches the variable's definition, and you incur the cost of destruction when the variable goes out of scope. This means there's a cost associated with unused variables, so you want to avoid them whenever you can.
Suave and sophisticated in the ways of programming as I know you to be, you're probably thinking you never define unused variables, so this Item's advice is inapplicable to your tight, lean coding style. You may need to think again. Consider the following function, which returns an encrypted version of a password, provided the password is long enough. If the password is too short, the function throws an exception of type logic_error
, which is defined in the standard C++ library (see Item 49):
// this function defines the variable "encrypted" too soon string encryptPassword(const string& password) { string encrypted; if (password.length() < MINIMUM_PASSWORD_LENGTH) { throw logic_error("Password is too short"); } do whatever is necessary to place an encrypted version of password in encrypted; return encrypted; }
encrypted
isn't completely unused in this function, but it's unused if an exception is thrown. That is, you'll pay for the construction and destruction of encrypted
even if encryptPassword
throws an exception. As a result, you're better off postponing encrypted
's definition until you know you'll need it:
// this function postpones "encrypted"'s definition until // it's truly necessary string encryptPassword(const string& password) { if (password.length() < MINIMUM_PASSWORD_LENGTH) { throw logic_error("Password is too short"); } string encrypted; do whatever is necessary to place an encrypted version of password in encrypted; return encrypted; }
encrypted
is defined without any initialization arguments. That means its default constructor will be used. In many cases, the first thing you'll do to an object is give it some value, often via an assignment. Item 12 explains why default-constructing an object and then assigning to it is a lot less efficient than initializing it with the value you really want it to have. That analysis applies here, too. For example, suppose the hard part of encryptPassword
is performed in this function:
void encrypt(string& s); // encrypts s in place
encryptPassword
could be implemented like this, though it wouldn't be the best way to do it:
// this function postpones "encrypted"'s definition until // it's necessary, but it's still needlessly inefficient string encryptPassword(const string& password) { ... // check length as above string encrypted; // default-construct encrypted encrypted = password; // assign to encrypted encrypt(encrypted); return encrypted; }
encrypted
with password
, thus skipping the (pointless) default construction:
// finally, the best way to define and initialize encrypted string encryptPassword(const string& password) { ... // check length string encrypted(password); // define and initialize // via copy constructor encrypt(encrypted); return encrypted; }
By postponing variable definitions, you improve program efficiency, increase program clarity, and reduce the need to document variable meanings. It looks like it's time to kiss those block-opening variable definitions good-bye.