Ada supports run-time checks in order to protect against access to unallocated memory, buffer overflow errors, off by one errors, array access errors, and other avoidable bugs. These checks can be disabled in the interest of efficiency, but can often be compiled efficiently. It also includes facilities to help program verification. For these reasons, it is very widely used in critical systems like avionics, weapons and spacecraft.
It also supports a large number of compile-time checks to help avoid bugs that would not be detectable until run-time in some other languages or would require explicit checks to be added to the source code.
Ada's dynamic memory management is safe and high-level, like Java and unlike C. The specification does not require any particular implementation.
Though the semantics of the language allow automatic garbage collection of inaccessible objects, most implementations do not support it. Ada does support a limited form of region-based storage management. Invalid accesses can always be detected at run time (unless of course the check is turned off) and sometimes at compile time.
The Ada language definition is unusual among ISO standards in that it is free content. One result of this is that the standard document (known as the Ada Reference Manual or ARM) is the usual reference Ada programmers resort to for technical details, in the same way as a particular standard textbook serves other programming languages.
In the 1970s, the US Department of Defense (DoD) was concerned by the number of different programming languages being used for its projects, many of which were obsolete or hardware-dependent, and none of which supported safe modular programming. In 1975 the Higher Order Language Working Group (HOLWG) was formed with the intent of reducing this number by finding or creating a programming language generally suitable for the department's requirements; the result was Ada. The total number of high-level programming languages in use for such projects fell from over 450 in 1983 to 37 by 1996.
The working group created a series of language requirements documents - the Strawman, Woodenman, Tinman, Ironman and Steelman documents. Many existing languages were formally reviewed, but the team concluded in 1977 that no existing language met the specifications.
Requests for proposals for a new programming language were issued and four contractors were hired to develop their proposals under the names of Red (Intermetrics led by Benjamin Brosgol), Green (CII Honeywell Bull, led by Jean Ichbiah), Blue (SofTech, led by John Goodenough), and Yellow (SRI International, led by Jay Spitzen). In April 1978, after public scrutiny, the Red and Green proposals passed to the next phase. In May of 1979, the Green proposal, designed by Jean Ichbiah at CII Honeywell Bull, was chosen and given the name Ada - after Augusta Ada, Countess of Lovelace. This proposal was influenced by the programming language LIS that Ichbiah and his group had developed in the 1970s. The preliminary Ada reference manual was published in ACM SIGPLAN Notices in June 1979. The Military Standard reference manual was approved on December 10, 1980 (Ada Lovelace's birthday), and given the number MIL-STD-1815 in honor of Ada Lovelace's birth year.
In 1987, the US Department of Defense began to require the use of Ada (the Ada mandate) for every software project where new code was more than 30% of result, though exceptions to this rule were often granted. This requirement was effectively removed in 1997, as the DoD began to embrace COTS (commercial off-the-shelf) technology. Similar requirements existed in other North Atlantic Treaty Organisation countries.
Because Ada is a strongly-typed language, it has been used outside the military in commercial aviation projects, where a software bug can mean fatalities. The fly-by-wire system in the Boeing 777 runs software written in Ada.
The language became an ANSI standard in 1983 (ANSI/MIL-STD 1815A, and without any further changes became an ISO standard in 1987 (ISO-8652:1987). This version of the language is commonly known as Ada 83, from the date of its adoption by ANSI, but is sometimes referred to also as Ada 87, from the date of its adoption by ISO.
Ada 95, the joint ISO/ANSI standard (ISO-8652:1995) is the latest standard for Ada. It was published in February 1995 (making Ada 95 the first ISO standard object-oriented programming language). To help with the standard revision and future acceptance, the US Air Force funded the development of the GNAT Compiler. Nowadays the GNAT Compiler is part of the GNU Compiler Collection.
Work continues on improving and updating the technical content of the Ada programming language. In October 2001, a Technical Corrigendum to Ada 95 was published. A major Amendment, ISO/IEC 8652:1995/Amd 1:2007, was published on March 9, 2007. This is the current version of the Ada standard. The next significant Ada Amendment is planned to be completed by 2012 (ISO/IEC 8652:201z Ed. 3). As of November 2011, work is on-going.
"Hello, world!" in Ada
A common example of a language's syntax is the Hello world program:
with Ada.Text_IO; procedure Hello is begin Ada.Text_IO.Put_Line("Hello, world!"); end Hello;
There are shortcuts available for Ada.Text_IO.Put_Line, needing less typing, however they are not used here for better understanding.
The Ariane 5 failure
A commonly encountered myth blames the loss of Ariane 5 Flight 501, a European Space Agency Ariane 5 rocket, on a bug in an Ada program or on disabling Ada's runtime checks. For the Ariane 4 it had been proven that those runtime checks weren't needed. Although range checks and appropriate exception handlers on all type conversions might have trapped the problem, the problem itself was a design decision to reuse a part and its software from the Ariane 4 rocket without adequate analysis of its suitability or tests on Ariane 5 data.