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In the C programming language, all of the language revisions I have worked with enforced up-front variable declarations before any non-declarative/assignative expressions would be evaluated. C++ seems to have waived this requirement from all versions. I also recognize more modern version of C have waived this requirement as well, but I have yet to use any of those standards.
The question I have is this: What historic reason was there for preventing the C language from declaring freely on-demand instead of up front?
Obviously there are a number of reasons that come to mind from an engineering standpoint, but none of them seem especially plausible to me.
Does anyone have any good sources backing up any of the above? Did I miss something entirely? We can speculate from dawn till dusk, but I'm looking for good hard references.
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Why do We Need to Declare a Variable in C? Basically, we need to declare a variable to store various types of data in the program. So to perform some operations or tasks with the data, we need to store them in the computer's memory location.
It's best to declare variables when you first use them to ensure that they are always initialized to some valid value and that their intended use is always apparent. The alternative is typically to declare all variables in one location, typically at the top of the block or, even worse, at the top of a function.
Before you can use a variable in C, you must declare it. Variable declarations show up in three places: Outside a function. These declarations declare global variables that are visible throughout the program (i.e. they have global scope).
Modern C compilers such as gcc and clang support the C99 and C11 standards, which allow you to declare a variable anywhere a statement could go. The variable's scope starts from the point of the declaration to the end of the block (next closing brace). You can also declare variables inside for loop initializers.
Looking at the early (6th edition Unix, 1975) C manual from Dennis Ritchie's home page, in that version function-local variables could only be declared at the beginning of a function:
The function-statement is just a compound statement which may have declarations at the start.
function-statement: { declaration-listopt statement-list }
declaration-list is not defined (an omission), but can be readily assumed to have grammar:
declaration-list: declaration declaration-listopt.
No other compound statement is allowed to contain variable (or indeed any) declarations.
This obviously simplifies the implementation; in the early compiler source code c02.c the function header function blkhed() only needs to sum the stack space used by auto variable declarations, at the same time recording their stack offset, and emit code to bump the stack pointer by the appropriate amount. On function exit (by return or falling off the end) the implementation just needs to restore the saved stack pointer.
The fact that K&R feels necessary to state that "declarations of variables (including initializations) may follow the left brace that introduces any compound statement, not just the one that begins a function" is a hint that at that point it was a relatively recent feature. It also indicates that combined declaration-initialization syntax was also a recent feature, and indeed in the 1975 manual declarators cannot have initializers.
The 1975 manual in section 11.1 specifically states that:
C is not a block-structured language; this may fairly be considered a defect.
Block-statement and initialized declarations (K&R) address that defect, and mixed declarations and code (C99) are the logical continuation.
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