How am I accessing the cpp standard library without placing '#include' statements?

Question

I've been teaching myself cpp sporadically from 'accelerated C++' and recently I noticed that when I forgot my #include <algorithm> statement, my code (which includes transform and find_if) compiled and ran successfully anyways. After this, I tried removing all standard header include statements altogether and found that my code still ran.

I assume my inability to understand preprocessor commands will be resolved by the time I finish the book, but for now I just need to know how to make sure that my terminal yells at me when I make the header improperly so I can learn where things are located in the std library.

I am running OS 10.6.5 so I have to compile my code with the following unix exe file:

CC = g++
CFLAGS = -Wall
PROG = TrainingProject23

SRCS = TrainingProject23.cpp

ifeq ($(shell uname),Darwin)
    LIBS = -framework OpenGL -framework GLUT
else
    LIBS = -lglut
endif

all: $(PROG)

$(PROG):    $(SRCS)
    $(CC) $(CFLAGS) -o $(PROG) $(SRCS) $(LIBS)

clean:
    rm -f $(PROG)

it includes the build protocol for OpenGL because I am learning that as well and it easy enough to use this file to compile all my C++ projects. I don't really understand the Makefile besides how to change the src file and program name, I just got it off the internet.

Answer 1

Standard library headers are allowed to include other standard library headers. So if you e.g. #include <string>; your implementation is allowed (but not required to) include every other standard library header there is, including <algorithm>. In your case, that probably happened, but it's nothing you should rely on.

Answer 2

It's hard to know for sure without seeing your exact code, but one possible option is that you are including other header files (Kristopher in comments is guessing OpenGL and/or GLUT) which in turn include the libraries your code didn't include explicitly.

E.g.

# File: my_incl.h
#include <algorithm>

# File: main.c
#include "my_incl.h"
# yay - you just included the algorithm.h without even trying. 

Please note that this works, but relying on it is Bad Practice, for a variety of reasons:

• If your project stops including "my_incl.h", it will suddenly stop compiling, without 100% immediately obvious cause.

• It makes the code harder to read/understand since a list of includes allows you a quick summary of which libraries are used by your code.

• It's Just Sloppy