Issue with main arguments handling

问题:

I can't compare main() arguments with const char* strings.

Simple code for explaining:

#include <stdio.h>

int main(int argc, char *argv[])
{
  int i;
  if(argc>1)
  {
    for (i=1;i<argc;++i)
    {
      printf("arg[%d] is %s\n",i,argv[i]);
      if(argv[i]=="hello")
        printf("  arg[%d]==\"hello\"\n",i);
      else
        printf("  arg[%d]!=\"hello\"\n",i);
    }
  }
  return 0;
}

Simple compile g++ test.cpp. When I try execute it, I see next thing:

>./a.out hello my friend
arg[1] is hello
  arg[1]!="hello"
arg[2] is my
  arg[2]!="hello"
arg[3] is friend
  arg[3]!="hello"

Whats wrong with my code?

回答1:

In this statement

if(argv[i]=="hello")

you compare pointers because the string literal is implicitly converted to const char * (or char * in C) that points to its first character. As the two pointers have different values the expression is always false. You have to use standard C function strcmp instead. For example

if( std::strcmp( argv[i], "hello" ) == 0 )

To use this function you should include header <cstring>(in C++) or <string.h> (in C).

回答2:

Strings can't be compared with ==, use strcmp:

if (strcmp(argv[i], "hello") == 0)

You have to #include <string.h>

回答3:

Whenever you use argv[i] == "hello", the operator "==" donot take string as its operand so in actual the compiler compares the pointer to argv[i] with the pointer to constant string "Hello" which is always false and hence the result you are getting is correct, to compare string literals use srtcmp function. int strcmp(const char *s1, const char *s2); which compares the two strings s1 and s2. It returns an integer less than, equal to, or greater than zero, if s1 is found, respectively, to be less than, to match, or be greater than s2.