Why do we need to specify type in format specifiers in the C++ printf function?

I've done a bit of PHP and when you declare variables you do not need to declare the type (eg. string or int).

However I have to tweak a very simple C file and, in order to get the print function to work correctly, I have to include a type specifier for the output of each variable.

Why isn't this type automatically inferred from the var开发者_运维问答iable itself?

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C is a very simple language, by which I mean it offers a pretty minimal abstractions over the bare metal. The basic types come with no type information attached and even structs only get what type info you give them.

So in a typical implementation, when the compiler sees

int x, y;
float z;
int x = foo(y,z);

x, y, and z are likely one machine word which is interpreted as a 2s-complement integer for x and `y, and as a IEEE-754 floating-point value for z, and nothing else.

The the compiler does something like

• advance the stack pointer to allow room for a return value
• push y and z onto the stack (what order it does this in is implementation dependent)
• push the current stack pointer+two instructions as the return address
• branch to the entry point of foo.

(Decent chips provide some support to amke this easier, but still...)

Then foo

• reads y and z from their locations relative the current stack value
• does whatever on them
• writes the return to x (again found by offset from the stack pointer)
• branch back to the return address that the calling routine pushed.

If foo has not been instructed what type the passed arguments have (as in a variadac routine like printf) it won't know how big each argument on the stack is (actually c coerces the size of every variadac argument with a set of up-conversion rules to fix this issue), nor how to interpret it.

PHP on the other hand, provides a sophisticated abstraction over the bare machine, and can (must, in fact to do what it does) carry some identifing information around with each value it works with.

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Because printf is part of the C-compatibility part of C++, where type detection wasn't so hot :-)

There is a disconnect between the specifiers in the format string and the data items being used for that specifier, for example:

printf ("String at memory %p is '%s'\n", mystr, mystr);
printf ("That character is '%c', ASCII code %d, hex %02x\n", mych, mych, mych);

If you use C++ streams for your output, this isn't so much of an issue.

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The most common specifiers are:

%d/%i is for an integer
%c is for char
%s is for a string
%f is for float/double

For example:

int i = 1;
char hi[6] = "Hello"
printf("%d %s", i, hi);

would print

1 Hello

as given above, you can refer http://www.cplusplus.com/reference/clibrary/cstdio/printf/