Prepone definitions? Wrong!

Before C99, you must prepone all the variables’ definitions before any statements. But after C99 was released, you can put a variable’s definition in any place as long as before being used. I think this new feature is introduced by learning from C++.

This makes programer write codes more convenient. But I used to insist the old style. I even teach my students that they should use the old style when I was a TA of the course Programming with C.

However, now I realize that I was wrong. We should postpone variables’ definitions as long as possible. It’s not for coding style, but for the effeciency of the program.

Consider this program.

#include <cstdio>
#include <iostream>

save_pwd(const std::string& pwd)
    std::string salted_pwd("abc");

    if (pwd.size() < 6) {
        fprintf(stderr, "Password too short.");
    salted_pwd += pwd;

main(int argc, char **argv)
    return 0;

When running this program, it will print an error and exit the function. The variable salted_pwd is not used. So, defining salted_pwd above the if statement will waste the memory, the time to allocate memory and the time to free memory. For the view of C++, you will pay for the cost of construction and destruction of the object salted_pwd.

Define and then assign? Wrong!

When I first learned programming in C, my teacher says that

int a;
a = 1;

is the same as

int a = 1;

Now I realize that they are not the same and the latter is more effecient.

For the former, int a; will allocate memory for the variable a and then fill zero bytes in the memory. Then a = 1; will write value 1 in the memory. For the latter, int a = 1; will directly allocate memory for the variable a and then write value 1 in the memory. In other words, you will pay more cost when using the former style.

It seems that it makes little difference with the built-in type. When the type of the variable a is user-defined type, great difference appears.

For example,

std::string str;
std::string = "abc";

This will first call the default constructor of std::string and then call the copy assignment operator. So the time and memory used in the default constructor is wasted.

If we define an object in this way

std::string str("abc");


std::string str = "abc";

, it will only call the copy constructor. This is more effecient.

So now I prefer this way

for (int i = 0; i < n; i++) {

instead of

int i;

for (i = 0; i < n; i++) {

Now there is a special case that is worth thinking.

Person p;
for (int i = 0; i < n; i++) {
    p = persons[i];


for (int i = 0; i < n; i++) {
    Person p = persons[i];

, which is better?

Let’s analysis the performance of them.

  • The former need 1 construction, n assignments and 1 destruction. But it makes the object p in a larger scope, which increase the comprehensibility and maintainability of the program.
  • The latter need n constructions and n destructions.

Which is more effecient? I think it all depends. If assignment is less expensive than the total of construction and destruction, we should use the former style. On the contray, we should use the latter style.

The author of Effective C++ prefers the latter, but I prefer the former, since I am sensitive with performance and I think memory allocation and free will be more expensive than assignment.