问题
Why is this ok:
if(int i = 1) {
}
...whereas the following produces errors?
if((int i = 1)) {
}
Under g++ (4.4.5) the latter gives:
test.cpp:7: error: expected primary-expression before ‘int’
test.cpp:7: error: expected ‘)’ before ‘int’
test.cpp:9: error: expected ‘)’ before ‘else’
test.cpp:13: error: expected primary-expression before ‘}’ token
test.cpp:13: error: expected ‘;’ before ‘}’ token
Incidentally, the reason I'm asking is because of this answer: Seeing what class an object is
I'm trying to find a way to make the condition more readable. Usually, I would prefer, for example:
if((x = y) != 0) {
to
if(x = y) {
...since it's more readable and silences compiler 'comments' suggesting I might have used the wrong operator. If I'm using a declaration as a condition, it doesn't produce the warning, but the readability still seems to suffer.
回答1:
It's because of the C++ standard, 6.4 p1.
Selection statements choose one of several flows of control.
selection-statement:
if ( condition ) statement
if ( condition ) statement else statement
switch ( condition ) statement
condition:
expression
type-specifier-seq declarator = assignment-expression
回答2:
This is not an assignment. It's a declaration. You can put declarations elsewhere as conditions, as in the following
if(int i = value) ...;
for(...; int i = value; ...) ...;
switch(int i = value) ...;
while(int i = value) ...;
This is a rarely used form, but it isn't an expression specified there. What you have done there is declaring a variable i which you can use in the body
// fire if get() returns non-null
if(weapon *w = get()) {
w->fire();
}
And parentheses aren't allowed around such a declaration. I think that would make no sense.
来源：https://stackoverflow.com/questions/4427228/declaration-as-condition-adding-parentheses-causes-errors