Think C (stylized as THINK C; formerly Lightspeed C) was an extension of the programming language C from the American National Standards Institute (ANSI C) for the classic Mac OS developed by Think Technologies, released first in mid-1986. The firm was later acquired by Symantec Corporation and the product continued to be developed by the original author, Michael Kahl. Versions 3 and later were essentially a subset of C++ and supported basic object-oriented programming (OOP) concepts such as single inheritance, and extensions to the C standard that conformed more closely to the needs of Mac OS programming. After version 6, the OOP facilities were expanded to a full C++ implementation, and the product was rebranded Symantec C++ for versions 7 and 8, then under development by different authors.
Think C, and later Symantec C++, featured a class library and framework for Mac programming called the Think Class Library (TCL), which was used extensively for Macintosh application development.
The Lightspeed and Think C integrated development environment (IDE) influenced other such environments, though considered not as advanced as that belonging to Think Pascal, its sister language product. It was considered the standard environment when Macintosh Programmer's Workshop (MPW) was considered an overpriced niche product, and most Macintosh products were developed in it for many years. However, with the transition of the Mac central processing unit (CPU) from the Motorola 68000 series (68K) to the PowerPC, Symantec was widely seen as having fallen behind, and competitor Metrowerks' product CodeWarrior took control of the market.
Despite the decline in popularity of their IDE, Symantec was eventually chosen by Apple to provide next-generation C/C++ compilers for MPW in the form of Sc/Scpp for 68K alongside MrC/MrCpp for PowerPC. These remained Apple's standard compilers until the arrival of Mac OS X replaced them with the GNU Compiler Collection (GCC). Symantec subsequently exited the developer tool business.
Bruce F. Webster of Byte named Lightspeed C product of the month for September 1986. While criticizing the documentation as its "single greatest weakness", Webster stated that Lightspeed C would be the choice if he had to select one compiler for the Macintosh. Byte in 1989 listed Lightspeed C as among the "Distinction" winners of the Byte Awards, stating that it "wins our respect because of its powerful features and low price".
- Denny, Bob (July 1986). "How the Chooser Works with AppleTalk". MacTech. Vol. 2 no. 7. Retrieved 2019-08-31., mentions Lightspeed C introduction, brief critique.
- Gordon, Bob (August 1986). "Menus and Windows in LightSpeed C". MacTech. Vol. 2 no. 8. Retrieved 2019-08-31.
- Dallas, Alastair (October 1989). "A First Look At Think C 4.0". MacTech. Vol. 5 no. 10. Retrieved 2019-08-31.
- Webster, Bruce F. (September 1986). "Two Fine Products". Byte. p. 335.
- "The Byte Awards". Byte. January 1989. p. 327.