
			/
			home
			/
			Programming
			/
			Style
			/

Auto

auto should only be used for Lambdas, Templates, and Macros.

Why not in ordinary code? ^§

auto costs time in the long run.
auto can be potentially dangerous.

1. Why does auto cost time in the long run? ^§

Code is read more often than it is written.
Explicit code is easiest to read.
auto is not explicit.
auto requires reader to deduce its type:


// Type is currently unknown
auto sneed = feed.seed();

// v Deduced from here:
QString Feed::seed() { ... }

// or possibly even here:  v
auto Feed::seed() { return 1; }

auto also requires reading right to left.


  int  sneed = 1;
// ^ type known immediately

  auto sneed = 1;
// ............^ type known @ EOL

auto is also prone to misdirection:


// char or int?
auto seed = '1';

// bool or string?
auto feed = "false";

2. How can auto be potentially dangerous? ^§

auto grants flexibility that can be exploited:


// Perfectly fine and acceptable
auto sneed = []( int  feed ){ return feed += 1024; };
sneed("seed"); // Will not compile!

// Dangerous and completely unacceptable
auto sneed = []( auto feed ){ return feed += 1024; };
sneed("seed"); // Buffer Overflow

// Output: "ceFrom == displaceTo"

Why use auto for a lambda's type? ^§

Lambdas have unique, unnamed class types.
Delegating their type is very difficult,
Except when there are no captures, a function pointer could be used:


// Function Pointer - Works
int (*sneed)() = [ ]() { return 1; };

// with capture - WILL NOT COMPILE!
int (*sneed)() = [&]() { return 1; };

// Acceptable either way.
auto sneed     = [ ]() { return 1; };
auto sneed     = [&]() { return 1; };

Why in Templates or Macros? ^§

Sometimes auto is needed to write otherwise impossible code:


union Seed;
union Feed
{
        int     operator+( Feed & ) { return 0     ; };
        QString operator+( Seed & ) { return "feed"; };
};
union Seed
{
        int     operator+( Feed & ) { return 1     ; }
        QString operator+( Seed & ) { return "seed"; }
};

// This template can now return either an integer, or a string
template<typename T0, typename T1> auto sneed()
{
        T0 p;
        T1 q;
        return p + q;
}
qDebug() << sneed<Feed,Feed>(); // 0
qDebug() << sneed<Feed,Seed>(); // "feed"
qDebug() << sneed<Seed,Feed>(); // 1
qDebug() << sneed<Seed,Seed>(); // "seed"


			|
			Home
			|
			About
			|
			Contact
			|